ApiParse: Clean up parsing code

ApiParse: Clean up parsing code

Now that ParserOptions->isSafeToCache() exists, use it where necessary.
This also moves the use inside the makeParserOptions() method so other
callers can pick it up as well.

Then pass the flag as $forceParse into WikiPage::getParserOutput()
instead of duplicating the logic in several cases, and generally clean
up the logic in the module to let WikiPage decide when to use the cache
in more cases.

Change-Id: I0079e10a40997e4a3b59ac21ef6c92246a147736