Add basic avatars and gravatar
Modularize get(), async getAvatar, avatars @ usermgmt

And other small improvements
Add unittests & check filetype in setLocalAvatar()

TODO: Fix OC_Image->mimetype(), it always returns "image/png"
Load avatar from path, if one's provided
Improve styling & enable avatar-upload at personal page
Have /avatar.php as a central avatar-point