POST actions can also be rendered

e.g.: 'reply' is a POST action; 'view' is a GET actions.
When submitting a reply, canSubmit will first be run (on 'reply'),
then the reply will be submitted, when we'll be redirected to 'view',
which will pass canRender.
When a reply fails, however (e.g. flagged by spamfilter), canRender
will be run on 'reply' (because we did not get redirected), which
will return false (reply is no GET action). This will cause a
InvalidActionException to be thrown (in View.php). Instead, we would
still like to actually render the output, which will include an
error message saying why the reply failed.

Change-Id: I317159a51eb31933a6265a52d8240e534d8bf580
Merge "POST actions can also be rendered"