Sanitize displayname, respect data @ $element, fix routename, clean after cropping, updateAvatar with displayname

Remove gravatar and no-avatar functionality, prepare for default avatars even more and reword some stuff

- Don't use gravatars default avatars
- Use "profile image" instead of "avatar"
- Use <p> instead of tables
- Ease updateAvatar()
- Actually return something in \OCP\Avatar

TODO: Fix OC_Image->mimetype(), it always returns "image/png"

And other small improvements

Fix #4491