Prefix Essentials featuring auto expiring prefixes and prefix filtering 2.5.4

Prefixes can be set to expire and on expiry, several actions can be applied to the thread.

• Delete thread (remove from public view, not permanent delete)
• Lock thread
• Move thread to a pre-defined, admin selected forum
• Assign a different, admin selected prefix (including none - i.e. remove the prefix)

- Fix prefix counts displayed in a forum did not respect "view threads by others" permission when counting viewable threads

Not a big deal, but thanks to @Robert9 for spotting the issue.

• Require php7+
• Require XF2.1+, remove XF2.0 support
• Fix additional query per-forum view when "Show Prefix group above the thread list for filtering" option is disabled
• Fix prefix page total not caching as expected
• Fix RSS support on a fresh XF2.2 install (or an XF2.1 install which has pruned old files)

Bug fix. Remove prefixes from the Ignored Prefixes list in User Preferences which

• Are not assigned to any forums
• Are assigned to non-visible forums.

• Performance fixes when a large number of threads exist on a prefix
• Implement short caching of thread totals per prefix as this can be quite slow
• Simplify viewing a list of threads for a prefix to only show visible threads (even if the visitor can see deleted/moderated threads)

Add caching to avoid performance impact of counting prefixes & threads every page load

Bug fix for an error in 2.0.3

XF2.1 compatibility update

Ensure latest thread prefix in forum list is updated when a thread's prefix is update

Fix saving a 'new posts' filter as the default filter