Add a mod log entry for replaced stickies

Previously, when stickying a post, the mod log would only include an
entry for the stickying of the new post, even if it replaced an existing
sticky. In the case of a replacement, this adds a second log entry
indicating which post was unstickied to make room for the new one. In
the case of accidentally stickying something, this can make it much
easier to locate the previously-stickied post.

r2/r2/models/subreddit.py

@@ -1186,7 +1186,9 @@ class Subreddit(Thing, Printable, BaseSite):
         return self.sticky_fullnames
 
     def set_sticky(self, link, user, num=None):
-        from r2.models import ModAction
+        from r2.models import Link, ModAction
+        unstickied_fullnames = []
+
         if not self.sticky_fullnames:
             self.sticky_fullnames = [link._fullname]
         else:
@@ -1197,6 +1199,7 @@ class Subreddit(Thing, Printable, BaseSite):
 
             # if a particular slot was specified and is in use, replace it
             if num and num <= len(sticky_fullnames):
+                unstickied_fullnames.append(sticky_fullnames[num-1])
                 sticky_fullnames[num-1] = link._fullname
             else:
                 # either didn't specify a slot or it's empty, just append
@@ -1204,6 +1207,8 @@ class Subreddit(Thing, Printable, BaseSite):
                 # if we're already at the max number of stickies, remove
                 # the bottom-most to make room for this new one
                 if len(sticky_fullnames) >= self.MAX_STICKIES:
+                    unstickied_fullnames.extend(
+                        sticky_fullnames[self.MAX_STICKIES-1:])
                     sticky_fullnames = sticky_fullnames[:self.MAX_STICKIES-1]
 
                 sticky_fullnames.append(link._fullname)
@@ -1211,6 +1216,11 @@ class Subreddit(Thing, Printable, BaseSite):
             self.sticky_fullnames = sticky_fullnames
             
         self._commit()
+
+        for fullname in unstickied_fullnames:
+            unstickied = Link._by_fullname(fullname)
+            ModAction.create(
+                self, user, "unsticky", target=unstickied, details="replaced")
         ModAction.create(self, user, "sticky", target=link)
 
     def remove_sticky(self, link, user):