[MODERATION] Refactor excluding watchers mechanism (squash)

This solves two bugs. One bug is that due to the JOIN with the
`forgejo_blocked_users` table, duplicated users were generated if a user
had more than one user blocked, this lead to receiving more than one
entry in the actions table. The other bug is that if a user blocked more
than one user, it would still receive a action entry by a
blocked user, because the SQL query would not exclude the other
duplicated users that was generated by the JOIN.

The new solution is somewhat non-optimal in my eyes, but it's better
than rewriting the query to become a potential perfomance blocker (usage
of WHERE IN, which cannot be rewritten to a JOIN). It simply removes the
watchers after it was retrieved by the SQL query.

(cherry picked from commit c63c00b39b)
(cherry picked from commit ad3cdc5705)

tests/integration/fixtures/TestBlockedNotifications/issue.yml

@@ -0,0 +1,16 @@
+-
+  id: 1000
+  repo_id: 4
+  index: 1000
+  poster_id: 10
+  original_author_id: 0
+  name: issue for moderation
+  content: Hello there!
+  milestone_id: 0
+  priority: 0
+  is_closed: false
+  is_pull: false
+  num_comments: 0
+  created_unix: 1705939088
+  updated_unix: 1705939088
+  is_locked: false