Tagging: Add support for filtering project TPs using tags

Fixes bug 3037
Tagging: Use Select2 in tag filtering UI

Fixes bug 3038
Tagging: Align the tags to progress bar column

Instead of letting the browser calculate the width of the tags cell, we
calculate it ourselves so that it does not change when whether tags are
visible or not.
Tagging: Use AJAX for filtering TPs in project overview

Fixes bug 3070
Tagging: Hide tags by default & save their visibility state
Tagging: Add a way for removing a tag from a TP

Fixes bug 3054
Tagging: Add a way for removing a tag from a TP in project overview

Fixes bug 3076
Tagging: Don't reload the whole page when adding a tag to a TP in Project overview

Fixes bug 3075
Tagging: Don't reload the whole page when adding a tag in TP overview

Fixes bug 3074