Make DesignManagement::Repository a container for GitRepository

What does this MR do and why?

This MR includes the code that was reverted in !117828 (merged) because of a regression (original MR: !116245 (merged)) and fixes the bug that caused the regression.

The original re-factor introduced a regression where designs could no longer be updated, uploaded or archived.

The last commit in this MR fixes that.

The re-factor makes DesignManagement::Repository the container for DesignManagement::GitRepository. But, RepoType#repository_for, RepoType#project_resolver and GitAccessDesign#user_can_push? expect the container to be of type project.

The fix is to select project as the container in those spots.

Another way to make it simpler would be to make the repository_resolver signature more like snippets. I am working on that in another MR, but Design Repo migration is a higher priority so this further work is stalled for the moment.

Please note Only the last commit needs a review, since the first commit was already reviewed in the original MR.

Related issue: #407752 (closed)

How to set up and validate locally

1. Design uploads

   a. Create a new issue or use an already existing one

   b. Upload a design - it should upload successfully

   c. Archive the design - it should archive successfully

2. Design replication

   a. Create an issue with a design on a primary

   b. 'Design repositories' should show '100%' on secondary at '/admin/geo/sites'

3. Move issue from one project to another - designs should move with the issue

   a. One easy way to do it is to use the move quick action in a comment on the issue to be moved: /move /path/to/new-project

MR acceptance checklist

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