Improve UI for site tags

[simonbaird]

trafficstars

Use https://bootstrap-tagsinput.github.io/bootstrap-tagsinput/examples/ or something like it.

[twMat]

I would also like to be able to edit the tags directly in the Your sites page, where they appear. Sometimes how you tag something depends on how other things are tagged - e.g what tags to reuse, tags to not use, how to formulate a variation on a tag (e.g a suffix) etc.

This is related to #76