Use special characters for `lfs+deploy-key` to prevent a someone from creating a user with this username, and method name refactoring.

Refactored LFS auth logic when using SSH to use its own API endpoint `/lfs_authenticate` and added tests.

- Required on the GitLab Rails side is mostly authentication and API related.

This reverts commit 13e37a3e.

Auth.find was a very generic name for a very specific method.
Auth.find_in_gitlab_or_ldap was inaccurate in GitLab EE where it also
looks in Kerberos.

Both work, but now we're consistent across the entire app.

Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

“Can create groups” and “Can create teams” had hardcoded defaults to
`true`. Sometimes it is desirable to prohibit these for newly created
users by default.

Should no longer freak out when omniauth settings aren't present in
gitlab.yml. People who aren't using it shouldn't even have to put a
'false' entry in their config for it (and probably wouldn't, after an
upgrade).