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
Cleanups
Tagging: properly delete tag slug in project overview

We need to remove the slug from the visible and the hidden tag list,
else it will show up again when the table is sorted.
Tagging: use AJAX to populate tags filter widget

This way the list of tags is always kept up to date as tags get added or
removed.
Tagging: fixup table filtering

This is a follow up of ff0127c6292cbe847699c0b9baae30f64444bdab, making
it more robust. Now selecting several tags, then removing them one by
one will re-filter the table each time, previously it was broken.
Tagging: Fix adding/removing tags when filtering is used

The tags would disappear/reappear when resetting the filtering, because
we were not updating the master table.