Merged ModelBase from devmo and sumo apps and get rid of cache machine.

This is an important step to have a leaner caching setup.
Added new "memcache" caching backend to be used by Jinja2 and constance.

Removed settings_local.py based CACHES setting in favor of settings.py.
This requires an update of all local_settings.py in use!
Move PageMoveError into exceptions module.
Extended render_stale_documents task to use a task group for its subtasks.

That means we can set a lock at the start of the task and destroy it again once the whole bunch of tasks have finished.
Added a MemcacheLock utility.
Use memcache cache backend for the rebuild task.
Merged the obscure lib/utils.py with devmo.utils.
Fixed the tests for the rebuilding of documents.
Stop deprecation warning to show up.
Minor cosmetic fixes.
Removed rebuild_kb task.
Merge branch 'master' into cache-lock-task-signals
Merge pull request #2423 from jezdez/cache-lock-task-signals

Re-enable memcache as a optional cache backend. Use it for locking.