Clean up settings file.

Production-only settings file has been removed. The file
local_settings.py is now imported on startup, enabling
customized local settings.
Remove a lot of files and libraries that doesn't need to be in the Git tree.

easy_thumbnails
mptt
Backup xml files
Some other small files I found
Merge branch 'master' of https://github.com/twhoffma/obs-osv-arkiv
More cleanup of settings file.