checkout 5.x-2.0

New features:
- Refactored large parts of the code.
- Moved node locking to an earlier stage to more reliably detect node edit urls
and avoid unnecessary locking. This also allows user defined urls to lock
nodes for (no UI yet).
- by ptalindstrom: Display error message even when the same user tries to edit
its own locked node.
- by Frank Steiner: (Backport) Trying to regain lock when a lock has been
removed.

Bugs fixed:
- #245685: Fixed endless page redirect if $base_url is set (ie. Drual is
installed in a subdirectory).
- Fixed node (un)locking, especially when using url aliases.