Improve LDAP user lookups

This patch simplifies configuration of user lookups in LDAP and also provides
better help text to aid the administrator setting it up.

The email thread below has more details:
https://groups.google.com/forum/#!topic/reviewboard-dev/J6W1o9Eb2IY

Some additional minor changes:
* I reflowed the anonymous bind and service account bind sections so that they
  always return a user dn which is authenticated in a common location.
* I dropped the useless fallback of joining the username with the base DN. It
  was unlikely to ever work in a real-world LDAP environment and it made the
  code confusing.
* Added a debug log message indicating the DN that we will attempt to
  authenticate against.
* Extended the get_or_create_user() routine to take the user DN Since we
  already have this information above, it doesn't make sense to search a second
  time. We'll just perform a BASE search instead.
* Corrected a typo in the help text of the Full Name Attribute

Testing Done:
Tested authenticating using TLS with and without using an LDAP service account
to perform the search.

Fixes bug 3004.

Reviewed at https://reviews.reviewboard.org/r/5203/