Another pass at a new Achievements settings page. WIP.
Correct filter name
Rename template pack function name for internal consistency
phpDoc corrections
Make settings method parameters mandatory

Quirk of bbPress code that was ported across.
phpDoc correction
For the _dpa_theme_package_id setting, use whitelist validation.
Word tweaks to new settings screen
i18n: add backpat for the old pre-3.6 'dpa' textdomain.
Clarify i18n context for some strings
Tweak strings on the settings screen again