[ticket/13263] Use prosilver as default style if user's style doesn't exist

If prosilver is not installed for whatever reason, it will be installed by
the newly added migration.

PHPBB3-13263
[ticket/13263] Only install/set prosilver if no style available

Users that have a nonexistent style selectd will revert back to the default
style.

PHPBB3-13263
[ticket/13263] Make sure default style exists and clean up code

PHPBB3-13263
Merge remote-tracking branch 'github-marc1706/ticket/13263' into prep-release-3.1.1

* github-marc1706/ticket/13263:
  [ticket/13263] Make sure default style exists and clean up code
  [ticket/13263] Only install/set prosilver if no style available
  [ticket/13263] Use prosilver as default style if user's style doesn't exist