Cleanup: Move RelatedManager to pootle.core.managers

All references or imports related to RelatedManager had been updated.

This allows us to get rid of pootle/apps/pootle_app/lib/.
More cleanups on properties

Remaining property() in codebase will be kept since they handle complex
situations or either using it the code is more clear.
License cleanups
Goals: Add initial infrastructure

This adds a goal tab for TPs and allows creating and removing goals using
the same form and links that can be used for tags.

Fixes bug 3073
Goals: Add management command for adding project goals from a file

Fixes bug 3092
WORKING: Goal overview page

Fixes bug 3094