FEATURE: flag dispositions normalization

All flags should end up in one of the three dispositions
  - Agree
  - Disagree
  - Defer

In the administration area, the *active* flags section displays 4 buttons
  - Agree (hide post + send PM)
  - Disagree
  - Defer
  - Delete

Clicking "Delete" will open a modal that offer to
  - Delete Post & Defer Flags
  - Delete Post & Agree with Flags
  - Delete Spammer (if available)

When the flag has a list associated, the list will now display 1
response and 1 reply and a "show more..." link if there are more in the
conversation. Replying to the conversation will NOT give a disposition.
Moderators must click the buttons that does that.

If someone clicks one buttons, this will add a default moderator message
from that moderator saying what happened.

The *old* flags section now displays the proper dispositions and is
super duper fast (no more N+9999 queries).

FIX: the old list includes deleted topics
FIX: the lists now properly display the topic states (deleted, closed,
archived, hidden, PM)
FIX: flagging a topic that you've already flagged the first post

app/serializers/topic_view_serializer.rb

@@ -173,7 +173,7 @@ class TopicViewSerializer < ApplicationSerializer
                   count: 0,
                   hidden: false,
                   can_act: scope.post_can_act?(post, sym)}
-      # TODO: other keys? :can_clear_flags, :acted, :can_undo
+      # TODO: other keys? :can_defer_flags, :acted, :can_undo
     end
     result
   end