Support renaming of achievement types

This more thoroughly addresses problems surrounding achievement type renaming, and reintroduces pieces from #271 that went missing. This rewrites the achievement type slug (preventing collision), updates all achievements of this type to use the new slug, updates all user meta for active and earned achievements, and outputs a custom message to keep admins informed about what a rename will mean for their site (http://d.pr/i/f7h5)

Fixes #26
Fixes #152  
Fixes #245
Fixes #267 
Fixes #282