No webhook notifications for tag create/delete events on the new code path

Context

Found in gitlab#359852 (closed).

Problem

We have identified that the registry is not emitting webhook notifications for events when those happen on the new code path.

This only happens when:

• Using the metadata database;
• The migration mode is enabled;
• Filesystem metadata mirroring is disabled;
• The repository is on the new code path.

This happens because the webhook notifications listener wraps filesystem metadata operations, so when running with filesystem metadata mirroring, none of the wrapped operations is executed. Therefore the outer notification wrapper isn't executed either and no notifications are sent.

Solution

Rails will start relying on manifest push (by tag) and tag deletion event notifications to trigger usage calculations at the project level, so it's key to restore these notifications right now. We should then fix all the remaining notifications with a broad refactoring in #691 (closed).