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.

-calling build_user will now apply defaults and only override them if as: :admin is set

Change-Id: Id1d938c0967752ecc14370af54f2d88128d18c44

“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.

fix: https://github.com/gitlabhq/gitlabhq/issues/1620