Relay some API methods to the actual submodule

Change-Id: I28bea47bb4aede9f2b2865353237473ac8c3eef6
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
(bug 61939) Topic-titles not displaying in some topics (disappearing?)

This will make sure results where content failed to be retrieved from
externalstore are filtered out.
Other than just displaying data that failed to load properly, this
data would also be cached. We don't want that ;)

Bug: 61939
Change-Id: Idbd223f1be72f8d207d707d9807e732a8c7b971a
Merge "POST actions can also be rendered"
Merge "Relay some API methods to the actual submodule"
Split out POST-specific logic from ApiFlowBase & add a new one for GET

Both should behave similarly, accepting input in the same way and
returning similar output.

This will make it easier to refactor ApiQueryFlow like what had been
done for the POST APIs.

Change-Id: Iaccc8ce7104d1c6a35477e432f58b0b2a06be722
Add ApiFlowViewHeader

Adds a new API module, similar to the existing ones, to request a
specific header.

Change-Id: I8ba4c0d453d28b324153afb0210d3eed03e1562a
Add ApiFlowViewTopicSummary

Adds a new API module, similar to the existing ones, to request a
specific topic summary.

Change-Id: Ic8d62378ff8c17bbb3c46e166868949ec1a98e41
Display a proper error when an invalid workflow is provided

Change-Id: Icaff730385cee286e7e68fb57f5be917e8227e5e
Merge "(bug 61939) Topic-titles not displaying in some topics (disappearing?)"
Merge remote-tracking branch 'gerrit/master' into frontend-rewrite

Change-Id: I60b5628f2872042ca48977be2f604eb4dd1712be