fix: restore manifest push (by tag) and tag delete webhook notifications

Partial fix for #682 (closed).

How to test this locally

Setup

1. Create two repositories by pushing an image to each. Ensure one is only present on the old code path (old/repo) and the other on the new code path (new/repo).

2. Go to https://requestbin.com/r and copy the endpoint URL on the top right.

3. Configure the registry as follows (truncated):

database:
  enabled: true
migration:
  enabled: true
  disablemirrorfs: true
notifications:
  endpoints:
    - name: pipedream
      url: <endpoint>
      timeout: 500ms
      threshold: 5
      backoff: 1s
      ignore:
        actions:
          - pull
      ignoredmediatypes:
        - application/octet-stream

Before

Using the master branch, start the registry and try:

1. Push an image to old/repo;
2. Delete tag in old/repo;
3. Push an image to new/repo;
4. Delete tag in new/repo.

You should only see notifications for (1) and (2) in pipedream.

After

Using this branch, repeat the steps above. You should see notifications for (3) and (4) as well.

Sample: https://requestbin.com/r/enibje2tcohfg/29Kyl55bV0KgcTqLIhR968qH7dh