Move the "You have comments in other revisions" blob into javascript.

This change drives the rendering of the info box for comments in other revisions
from javascript instead of HTML. This is the last piece of the diffviewer
needed before I can do dynamic updates.

Testing done:
* Created draft comments on several diff and interdiff revisions and verified
  that it appeared correctly.
* Tested in conjunction with my change for dynamic updates.
* Ran unit tests.

Reviewed at https://reviews.reviewboard.org/r/4626/