New License model to use for exercises, ingredients, etc.

Also add necessary views to manage the licenses and update or add tests.
Use License model to explicitly set a license for exercises

--HG--
rename : wger/manager/templates/tags/cc-by-sa-sidebar.html => wger/manager/templates/tags/license-sidebar.html
Use License model to explicitly set a license for ingredients
Expose and allow filtering of ingredient language throut the REST api
Cleanup unused imports
Use License model to set licenses for exercise images
Allow filtering the exercise REST endpoint by license information
Remove the 'user' field from ingredients and use the license author instead
Correctly set the license author for exercise images
Fix tests
Remove cc_by_sa template tags
Further reduce the tested length of generated PDF
Update API documentation with the new license endpoint
Remove .orig file that was commited by mistake
Optimize and simplify migration dependencies and installation steps
Remove the banner for submitted exercises, info is in license box
Merge pull request #66 from rolandgeider/license

Use common way to set licenses and author information with foreign keys