Move output functionality from AbstractRevision to Templating

AbstractRevision::getUserText() & ::getContent() had some functionality that did
not seem like it belonged there. I feel like AbstractRevision should return the
exact data & Templating (responsible for output) should deal with how it should
be displayed, depending on the moderation status.

Since there were multiple getUserText (getUserTextRaw) and getContent
(getConvertedContent, unused getHiddenContent) methods to fetch different
variations of usertext/content, I think it makes sense to keep the "pure data"
in AbstractRevision & the "how do we output it for a certain user and moderation
level" in Templating.

I've removed the now pointless AbstractRevision::getConvertedContent,
::getHiddenContent & ::getUserTextRaw, in favor or just getContent &
getUserText, which will always return raw data.

Added/re-instated Templating::getContent() & ::getUserText(), which should be
used when outputting that data.

Got rid of AbstractRevision::$perms[*]['content'] & ['usertext'] in
AbstractRevision; didn't belong there. Strings are now simply
flow-$state-usertext * flow-$state-content, as defined in Templating.

Note: I have, for now, ignored Notifications & RecentChanges. Both of these are
saved somewhere else right away, so won't be moderated at that time yet.
We should think about this some day.

Change-Id: Ie42a5b1c6efd618a5bd2fa1e7ac43316c239cb50