Avoid extra parse/save delay for users with non-canonical parser options

Avoid extra parse/save delay for users with non-canonical parser options

If {{REVISIONID}} results in a re-parse, that re-parse will be post-send
unless the user has canonical parser options and will need the output for
page views anyway (e.g. the refresh after editing).

Also make getPreparedEdit() allow lazy-loading of the parser output via
a callback. A magic __get() method handles objects created the new way
but accessed by other code the old way.

Bug: T216306
Change-Id: I2012437c45dd605a6c0868dea47cf43dc67061d8