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

Speedup DiffNote#active? on discussions, preloading noteables and avoid touching git repository to return diff_refs when possible

- Preloading noteable we share the same noteable instance when more than one 
discussion refers to the same noteable.
- Any other call to that object that is cached in that object will be for any 
discussion.
- In those cases where merge_request_diff has all the sha stored to build a diff_refs get that 
diff_refs using directly those sha instead accessing to the git repository to first get the 
commits and later the sha.

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       |

NotesHelper#note_editable? and ProjectTeam#human_max_access currently
take about 16% of the load time of an issue page. This MR preloads
the maximum access level of users for all notes in issues and merge
requests with several queries instead of one per user and caches
the result in RequestStore.

This reverts commit c3935699.

1. Move render method to a concern, not a helper.
2. Let DiffHelper#diff_options automatically add the path option.
3. Move more instance var definitions to before filters.

When rendering a list of diff files, skip those where the diff is over
10 KB and provide an endpoint to render individually instead.

We just need to get merge requests closes issues when 
we’re going to show them

Extract diff line data attributes to helper methods

Depends on the changes in Workhorse (gitlab-org/gitlab-workhorse!48).

Fix a wrong MR status when merge_when_build_succeeds & project.only_allow_merge_if_build_succeeds are true

The issue was that `MergeRequest#mergeable?` returns false when the CI
state is not success and project.only_allow_merge_if_build_succeeds is
true. In this case `Projects::MergeRequestsController#merge` would
return the `:failed` status when enabling `merge_when_build_succeeds`,
thus leading to a weird state and the MR never automatically merged.

The fix is to disable the CI state check in the controller safeguard
that early return the `:failed` status.
Signed-off-by: NRémy Coutable <remy@rymai.me>

This commit changes the way certain documents are rendered (currently
only Notes) and how documents are redacted. Previously both rendering
and redacting would run on a per document basis. The result of this was
that for every document we'd have to run countless queries just to
figure out if we could display a set of links or not.

This commit changes things around so that redacting Markdown documents
is no longer tied into the html-pipeline Gem. This in turn allows it to
redact multiple documents in a single pass, thus reducing the number of
queries needed.

In turn rendering issue/merge request notes has been adjusted to take
advantage of this new setup. Instead of rendering Markdown somewhere
deep down in a view the Markdown is rendered and redacted in the
controller (taking the current user and all that into account). This has
been done in such a way that the "markdown()" helper method can still be
used on its own.

This particular commit also paves the way for caching rendered HTML on
object level. Right now there's an accessor method Note#note_html which
is used for setting/getting the rendered HTML. Once we cache HTML on row
level we can simply change this field to be a column and call a "save"
whenever needed and we're pretty much done.

This reverts commit 13e37a3e.

Added a button into the sidebar for issues & merge requests to allow users to manually create todo items

Closes #15045

This reverts commit 3e991230.

# Conflicts:
#	app/models/project.rb