Alert users when forum AJAX requests fail

AJAX requests on forums pages occasionally fail (usually when a request
to the comments service times out), but the user is not made aware of
the failure. This manifests as either the system not appearing to store
state (e.g. if an attempt to flag a post fails) or something taking
forever to load. Now, a modal will pop up to alert the user that a
request has failed and instruct them to reload the page.

In the longer term, we should fix each user gesture that results in an
AJAX call to gracefully handle a failure by resetting the state of the
world appropriately and aleritng the user.

JIRA: FOR-37