This reverts commit 2536fea8db0967a817615eb3e951eb6ba22f8ede.

An MR can only be resolved in the UI if:
- It has conflicts.
- It has valid diff_refs (in other words, it supports new diff notes).
- It has no conflicts with one side missing.
- It has no conflicts in binary files.
- It has no conflicts in files too large to display.
- It has no conflicts containing invalid conflict markers.

This change simplifies a Pipeline processing by introducing a special new status: created.
This status is used for all builds that are created for a pipeline.
We are then processing next stages and queueing some of the builds (created -> pending) or skipping them (created -> skipped).
This makes it possible to simplify and solve a few ordering problems with how previously builds were scheduled.
This also allows us to visualise a full pipeline (with created builds).

This also removes an after_touch used for updating a pipeline state parameters.
Right now in various places we explicitly call a reload_status! on pipeline to force it to be updated and saved.

Before this change we always let users push Git data over HTTP before
deciding whether to accept to push. This was different from pushing
over SSH where we terminate a 'git push' early if we already know the
user is not allowed to push.

This change let Git over HTTP follow the same behavior as Git over
SSH. We also distinguish between HTTP 404 and 403 responses when
denying Git requests, depending on whether the user is allowed to know
the project exists.

So we have raw_diffs too

This object will manage Gitlab::Git::Compare instances

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.

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.

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       |

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