Move percentage functions to pootle_misc.stats

Since they are used to work on stats it makes more sense to have them there.
Refactor nice_percentage() to make it calculate the percentage

This simplifies code when calling it.
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