Add comment linking to Maniphest and Differential

Summary:
Allows you to link to comments with "D123#3" or "T123#3", then adds a pile of JS
to try to make it not terrible. :/

The thing I'm trying to avoid here is when someone says "look at this!
http://blog.com/#comment-239291" and you click and your browser jumps somewhere
random and you have no idea which comment they meant. Since I really hate this,
I've tried to avoid it by making sure the comment is always highlighted.

Test Plan:
Put T1#1 and D1#1 in remarkup and verified they linked properly.

Clicked anchors on individual comments.

Faked all comments hidden in Differential and verified they expanded on anchor
or anchor change.

Reviewed By: aran
Reviewers: aran, tomo, mroch, jungejason, tuomaspelkonen
CC: aran, epriestley
Differential Revision: 383

src/docs/userguide/remarkup.diviner

@@ -98,6 +98,10 @@ by mentioning the name of an object:
                 # You must specify at least 7 characters of the hash.
   T123          # Link to Maniphest task T123
 
+You can also link directly to a comment in Maniphest and Differential:
+
+  T123#4        # Link to comment #4 of T123
+
 = Quoting Text =
 
 To quote text, preface it with an ">":