We've seen a significant performance penalty when using
`BatchLoader#__replace_with!`. This defines methods on the batch loader
that proxy to the 'real' object using send. The alternative is
`method_missing`, which is slower.  However, we've noticed that
`method_missing` can be faster if:

1. The objects being loaded have a large interface.
2. We don't call too many methods on the loaded object.

Avatar uploads meet both criteria above, so let's use the newly-released
feature in https://github.com/exAspArk/batch-loader/pull/45.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/60903

In a merge request with many discussions, the avatar URL can be called
thousands of times, inflicting a significant performance penalty
especially when avatars are stored in object storage. To mitigate this
problem, we can just cache the generated path any time it is requested.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/55355

Every time a column in the projects table is changed, the Avatarable concern
would validate that the avatar file size was under 200K.  This not only delays
the database changes, but it also can lead to unrelated failures if the HTTP
request fails for some reason.

Closes #51053

Partially addresses #47424.

This is tricky: the query was being run in
`ObjectStorage::Extension::RecordsUploads#retrieve_from_store!`, but we can't
just add batch loading there, because the `#upload=` method there would use the
result immediately, making the batch only have one item.

Instead, we can pre-emptively add an item to the batch whenever an avatarable
object is initialized, and then reuse that batch item in
`#retrieve_from_store!`. However, this also has problems:

1. There is a lot of logic in `Avatarable#retrieve_upload_from_batch`.
2. Some of that logic constructs a 'fake' model for the batch key. This should
   be fine, because of ActiveRecord's override of `#==`, but it relies on that
   staying the same.

Bring Object Storage to CE

Closes #4171, #4163, #3370, #2841, and #29203

See merge request gitlab-org/gitlab-ce!17358