Add manage_mediawiki_php_symlink config flag

scap deploy-promote (via scap update-wikiversions) updates the
/srv/mediawiki-staging/php symlink to point to the latest train branch
directory every times it runs. Generally this gets committed to
operations/mediawiki-config during the automated train presync to
testwikis. In addition, scap deploy-promote specifically syncs the
/srv/mediawiki-staging/php symlink after promoting to group1 (even
though it will already have been sync'd during presync to testwikis).

The new manage_mediawiki_php_symlink config flag can be set to False
to disable these behaviors.

Bug: T359643