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

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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
Add workout sessions from the calendar view to the selected date
Fix button label in user preferences page
Update jQueryMobile to 1.4.1
After saving the workout session from timer view, disable the button
Don't allow search engines to follow the links that switch between versions

Most engines have different versions anyway and the mobile one will get the
mobile version.
Allow workout logs to be edited and deleted from calendar views
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