Start writing comment participation by thread.

This new DenormalizedRelation tracks when a user is the author of a
comment in the given thread.  Once it's backfilled, we can use it to
quickly determine eligibility for the CommentPane cache.

r2/r2/models/link.py

@@ -735,6 +735,8 @@ class Comment(Thing, Printable):
 
         c._commit()
 
+        CommentParticipationByAccount.register(author, link)
+
         changed(link, True)  # link's number of comments changed
 
         inbox_rel = None
@@ -1382,6 +1384,20 @@ class SaveHide(Relation(Account, Link)): pass
 class Click(Relation(Account, Link)): pass
 
 
+class CommentParticipationByAccount(tdb_cassandra.DenormalizedRelation):
+    _use_db = True
+    _last_modified_name = 'Commented'
+    _write_last_modified = False
+    _views = []
+
+    @classmethod
+    def value_for(cls, thing1, thing2, opaque):
+        return ''
+
+    @classmethod
+    def register(cls, user, link):
+        cls.create(user, [link])
+
 
 class GildedCommentsByAccount(tdb_cassandra.DenormalizedRelation):
     _use_db = True