Multi Prefix 2.16.1

This add-on is no longer in beta and can be considered feature complete and stable.

Changes from beta:

• Fixup single prefix selection to include a 'none' option.
• Rework detection of prefixes a user is not permitted to remove

• On quick post thread forum, fix displaying empty prefix input when no prefixes are usable.
• Add descriptions to template modifications
• Move multiprefix filter and quick post thread template chunks to their own template

• Make quick-thread feature only show prefix selection once post-thread form is expanded
• Ensure prefix order is forced to match prefix display order to prevent unexpected flip-flopping of prefixes changing.
  • The front-end currently submits prefixes in display order, this ensures the backend enforces that.
• Improve reliability of some template modifications
• Improve uninstaller not removing all extra columns from some tables.
• Improve installer upgrade code path to handle pre-1.5.0 versions

• Fix upgrading from pre-1.6.x
• Switch to select-toggle mode when only 1 max prefix is permitted
• When moving a resource/thread to a forum which supports less prefixes than currently selected, trigger the pop-up informing the user that less prefixes are permitted

• Fix creating threads/resources without a prefix