feat: Follow remote users; feed tab (#10380)

This is hopefully the final part of PR #4767, rebased and squashed.

More thorough federation tests are at https://code.forgejo.org/forgejo/end-to-end/pulls/1276 but the mock has been extended to hopefully cover a good chunk as well.

Co-authored-by: Gergely Nagy <forgejo@gergo.csillger.hu>
Co-authored-by: Michael Jerger <michael.jerger@meissa-gmbh.de>
Co-authored-by: zam <mirco.zachmann@meissa.de>
Co-authored-by: Panagiotis "Ivory" Vasilopoulos <git@n0toose.net>

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/10380
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
Co-authored-by: famfo <famfo@famfo.xyz>
Co-committed-by: famfo <famfo@famfo.xyz>

.deadcode-out

@@ -88,7 +88,6 @@ forgejo.org/modules/eventsource
 	Event.String
 
 forgejo.org/modules/forgefed
-	NewForgeFollow
 	NewForgeUndoLike
 	ForgeUndoLike.UnmarshalJSON
 	ForgeUndoLike.Validate
@@ -227,9 +226,6 @@ forgejo.org/routers/web/org
 forgejo.org/services/context
 	GetPrivateContext
 
-forgejo.org/services/federation
-	FollowRemoteActor
-
 forgejo.org/services/notify
 	UnregisterNotifier