[bug 1030532] Needs change comment now cleared.

When the box is unchecked and the form is submitted, the comment
is now cleared.
Merge pull request #2005 from dean/revision-persist-1030532

[bug 1030532] Needs change comment now cleared.
[bug 1006574] Top contributors infrastructure.
[bug 1006574] Implement initial UI for top contributors.

* Implemented the top contributors view and template
* Implemented the top contributors on the Community Hub landing page
[bug 1030348] Fixes issue with Topic selection Dropdown
[bug 971782] JS fix for data bucketing (weekly, monthly).

The bucketing implemented in Rickshaw does not like missing data
for any dates. It wants explicit 0s. This zero-fills all the
dates.
[bug 987337] Show topic descriptions on topic pages.
[bug 1032175] Update error message
[Bug 1031906] Update error message in users/forms.py
[bug 1001145] Track usage for canned responses