GetFeeds must always discard actions with dangling repo_id (#19598) (#19629)

Co-authored-by: Loïc Dachary <loic@dachary.org>
(cherry picked from commit b536b65189)

Conflicts:
        models/action_test.go
	  The GetFeeds function does not have a Context argument in 1.16.
	models/action.go
	  The SQL statement is essentially the same in 1.16 but
	  structured differently. The Join() was copied and the
   	  created_unix field prefixed with `action`.
	models/action_list.go
	  in 1.16 the loadRepoOwner method did not exist and
	  it was done in the RetrieveFeeds method of web/feed/profile.go.
          The safeguard to skip when act.Repo == nil was moved there.

models/fixtures/action.yml

@@ -56,3 +56,11 @@
   repo_id: 8
   is_private: false
   created_unix: 1603011540 # grouped with id:7
+
+- id: 8
+  user_id: 1
+  op_type: 12 # close issue
+  act_user_id: 1
+  repo_id: 1700 # dangling intentional
+  is_private: false
+  created_unix: 1603011541