Instead calling diff_collection.count use diff_collection.size which is cache on the diff_collection

Introducing the concept of SafeDiffs which relates 
diffs with UI highlighting.

Now we link to the standard `IssuesController#new` action, and let it
redirect if we're using an external tracker.

Prior, in order to display the correct link to "Issues" in the project
navigation, we were performing a check against the project to see if it
used an external issue tracker, and if so, we used that URL. This was
inefficient.

Now, we simply _always_ link to `namespace_project_issues_path`, and
then in the controller we redirect to the external tracker if it's
present.

This also removes the need for the url_for_issue helper. Bonus! 🎉

Provide more sensible default sort order for issues and merge requests
based on the following table:

| type           | state  | default sort order |
|----------------|--------|--------------------|
| issues         | open   | last created       |
| issues         | closed | last updated       |
| issues         | all    | last created       |
| merge requests | open   | last created       |
| merge requests | merged | last updated       |
| merge requests | closed | last updated       |
| merge requests | all    | last created       |

On a merge request with over 1000 changed files, there were redundant
calls to blob_text_viewable?, which incurred about 7% of the time.

Improves #14775

This MR adds a string (thus max 255 chars) field to the enviroments
table to expose it later in other features.

1. Instantiate `ProtectedBranchesAccessSelect` from `dispatcher`

2. Use `can?(user, ...)` instead of `user.can?(...)`

3. Add `DOWNTIME` notes to all migrations added in !5081.

4. Add an explicit `down` method for migrations removing the
   `developers_can_push` and `developers_can_merge` columns, ensuring that
   the columns created (on rollback) have the appropriate defaults.

5. Remove duplicate CHANGELOG entries.

6. Blank lines after guard clauses.

1. It makes sense to reuse these constants since we had them duplicated
   in the previous enum implementation. This also simplifies our
   `check_access` implementation, because we can use
   `project.team.max_member_access` directly.

2. Use `accepts_nested_attributes_for` to create push/merge access
   levels. This was a bit fiddly to set up, but this simplifies our code
   by quite a large amount. We can even get rid of
   `ProtectedBranches::BaseService`.

3. Move API handling back into the API (previously in
   `ProtectedBranches::BaseService#translate_api_params`.

4. The protected branch services now return a `ProtectedBranch` rather
   than `true/false`.

5. Run `load_protected_branches` on-demand in the `create` action, to
   prevent it being called unneccessarily.

6. "Masters" is pre-selected as the default option for "Allowed to Push"
   and "Allowed to Merge".

7. These changes were based on a review from @rymai in !5081.

1. Align "Allowed to Merge" and "Allowed to Push" dropdowns.

2. Don't display a flash every time a protected branch is updated.
   Previously, we were using this so the test has something to hook onto
   before the assertion. Now we're using `wait_for_ajax` instead.

- Based on feedback from @axil
- http://docs.gitlab.com/ce/development/ui_guide.html#buttons

1. The model now contains this humanization data, which is the once
   source of truth.

2. Previously, this was being listed out in the dropdown component as well.

1. Get the existing spec passing.

2. Add specs for all the access control options, both while creating and
   updating protected branches.

3. Show a flash notice when updating protected branches, primarily so
   the spec knows when the update is done.

1. Reuse the same dropdown component that we used for updating these
   settings (`ProtectedBranchesAccessSelect`). Have it accept options
   for the parent container (so we can control the elements it sees) and
   whether or not to save changes via AJAX (we need this for update, but
   not create).

2. Change the "Developers" option to "Developers + Masters", which is
   clearer.

3. Remove `developers_can_push` and `developers_can_merge` from the
   model, since they're not needed anymore.

1. Move to dropdowns instead of checkboxes. One each for "Allowed to
   Push" and "Allowed to Merge"

2. Refactor the `ProtectedBranches` coffeescript class into
   `ProtectedBranchesAccessSelect`.

3. Modify the backend to accept the new parameters.

Before when you choose the way of `sort` instead it display the title correctly it was just apply the humanize helper in sort value.

E.g.

When you choose `Last updated` it should display the title `Last updated` instead of `Recently updated`. This fix makes this correctly displays the title.

Change the implementation of the `link_to` `filter_branches_path`

- Change the value of the `params[:sort]` in `link_to`. E.g. instead of using `'recently_updated'` is now using `sort_value_recently_updated`.

- Change the values of the case in the `branches_sorted_by` method for the values it receives in the `params[:sort]` that are: `nil`, `'name'`, `'updated_desc'`, `'updated_asc'`.

We can't reuse the existing value of the format parameter, because on
the merge request page that's 'json', so the link would go to a JSON
file. We can't set it to HTML, because that adds '.html' the URL, which
breaks the compare view (and looks bad). Setting it to nil uses the
default format, which in all of these cases is HTML anyway.

Signed-off-by: NRémy Coutable <remy@rymai.me>

For both having existing issues and having no issues.