(bug 44243) Avoid fatal error for non-text content.

AFComputedVariable::compute() caused fatal errors when trying to handle
non-text content, e.g. in Special:AbuseLog.

For now, we just stub out some variables for non-textual content. Extensions
can still provide values for these using the AbuseFilter-interceptVariable hook.
A real solution requires us to make the Content object available along with the
variable array, or at the very least log the base revision id of filtered changes.

Change-Id: I795549b733f2f418b471223786d206fd8925d8fc
Merge "(bug 44243) Avoid fatal error for non-text content."