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

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