Disable expensive {{REVISIONID}} magic word in miser mode

Disable expensive {{REVISIONID}} magic word in miser mode

This only applies to content namespaces for now since
the cost of vary-revision-id is much less of a concern.

The potential to harm page save time is far worse than what
use they have, which is almost entirely just hacks to check
for preview mode. These have nothing to do with the actual
revision ID nor timestamp itself. They simply check whether
the value is the empty string. Since this magic word still
only returns an empty string in preview mode, such checks
will keep working.

Bug: T137900
Depends-on: I1809354055513a5b9d9589e2d6acda7579af76e2
Change-Id: Ieff8423ae3804b42d264f630e1a029199abf5976