Before we started working on group wikis, `ProjectWiki` used a `Project`
instance as the container of its `Repository` instance, so for group
wikis we did the same with `Group`.

This initially made sense and also mostly aligned with the semantics for
`Repository#container`, but while working on wiki diffing [1] we noticed
that the `Commit` and `Blob` classes sometimes use
`container#repository`, which breaks some assumptions and also causes
redundant Gitaly calls to the project repository when diffing wikis.

Refactoring those classes is a lot riskier and would affect other
features too, so in this commit we're instead changing the
`Repository#container` to be the wiki instance, rather than the project
or group. This generally seems to make sense anyway, and only needs some
small adjustments.

To satisfy the interface for `Repository#container`, we need to add
or tweak some methods:

- `Wiki.find_by_id`
- `Wiki#id`
- `Wiki#to_global_id`

We also still need to be able to resolve wiki repositories from their
containers, this is handled in the repository resolver for
`Gitlab::GlRepository::WIKI`.

[1] https://gitlab.com/gitlab-org/gitlab/-/merge_requests/35330