Docs: Fix various issues with gaols page

Fix issues identified in the review of
https://github.com/translate/pootle/commit/133de338580
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.