Improve LDAP authentication

* Adds an LDAP filter for locating the LDAP object
* Adds a search scope policy when using the Base DN
* Adds option required certificate policy when using LDAPS
* Adds attribute mapping for username, firstname, lastname, email
* Initializes rhodecode user using LDAP info (no longer uses "@ldap")
* Remembers the user object (DN) in the user table
* Updates admin interfaces
* Authenticates against actual user objects in LDAP
* Possibly other things.

Really, this should be extended to a list of LDAP configurations, but this is a good start.