Rework ldap integration with ldap3

Use the pure-python ldap3 library, which eliminates the need for a
system dependency.

Offer both a `search` and `simple_bind` mode, for more sophisticated
ldap scenarios.
- `search` tries to find a matching DN within the `user_base` while
  employing the `user_filter`, then tries the bind when a single
  matching DN was found.
- `simple_bind` tries the bind against a specific DN by combining the
  localpart and `user_base`

Offer support for STARTTLS on a plain connection.

The configuration was changed to reflect these new possibilities.

Signed-off-by: Martin Weinelt <hexa@darmstadt.ccc.de>