LDAP Object Mapper

Replacing the hand-written User/Group/Mail classes with a generic SQLAlchemy-like ORM for LDAP to simplify the application code.

The work on the LDAP ORM functionality is complete. The code now lives in a separate repository. It still lacks unit tests and documentation, but the interface will probably not change much anymore.

The port of the uffd codebase still has two problems:

• LDAP_USER_ATTRIBUTE_EXTRA is not implemented and was removed from default config
• LDIF output on user page is not implemented (dummy tab still exists in template)

Both problems are a lot more complicated to solve due to the ORM and I don't think they are worth the effort/added code complexity.

Aside from the problems the uffd code could be refactored a lot more with the new abstractions, but I don't plan to do this (for now).

While I would not merge this until the ORM code has at least basic tests, the uffd port is ready for a thorough code review. I'm also interested in opinions on the mentioned problems.