Add setting to resolve outdated diff comments on push

Description

Adds a new project setting for MRs that, when set, automatically resolved outdated diff discussions on a push to an MR.

When the setting is enabled:

Expanded:

Original description

In an MR, GitLab will only collapse comments on diffs that have been marked as resolved unlike on GitHub where they are automatically collapsed when the diff becomes outdated. There is no setting in repositories to toggle this functionality.

For some developer workflows that don't rely as heavily on discussions, it would be nice to have a repository setting that would make it so comments would automatically collapse like on GitHub. This makes sense for when developers address the comments through code in a follow-up commit thus outdating the comment(s) on the previous commit. GitLab once behaved like this but the feature was removed. The discussion can remain as unresolved since it's possible to merge a MR even with unresolved discussions assuming the appropriate toggle is set on the repo.

Links / references Relates to https://gitlab.com/gitlab-org/gitlab-ce/issues/17973

Use cases

This is for teams that don't make as heavy use of the discussion feature.