Update ACP icons and images

The ACP icons needed updating to PNG images. For consistency with the frontend, I've used the Fugue icons. I've also removed some unneeded images and CSS. There are still a few images which I haven't updated as I couldn't find a suitable replacement.
Update usergroups.xml version code
Drop `useragent` column before upgrading
Update Standards
Add ability to rebuild/reload all caches

MyBB provides the ability to rebuild a single cache, one at a time. It is often desirable to rebuild all caches (for example, when you switch to a new cache handler).
Update Standards
Update version check task
Revert "Double-escaping the user's data and enhanced the website verification enhancement."

This was reverted in the stable branch but needs to be reverted in the feature branch too. Other avenues will be explored to improve these areas.
Remove `text-decoration: underline` from ACP links

These look woefully out of place on the new theme.
Update memberlist_user_avatar to use default avatar
Update portal_announcement_avatar to use default avatar
Remove setting blocking CSS

MyBB has a "Maximum Avatar Dimensions in Posts" setting, but the new 1.8 theme has CSS which blocks this functionality.
Merge pull request #98 from PaulBender/feature

Fixes #2250 Admin Log errors
Make Recount & Rebuild use the language system
Update post icons
Update "Send Thread to a Friend" wording
Open "What is RSS?" link in new tab
Revert "Double-escaping the user's data and enhanced the website verification enhancement."
Update expandables images
Update "Maximum Avatar Dimensions in Posts" setting

This was styled with CSS but the setting no longer took effect. Reduce the size to 55x55.