Prefix Essentials 2.1.0

Change log

- 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)

Release notes

Performance improvement and bug fixes.