Remove key:value features of UserTag

[jywarren]

This is a complex issue that needs to be broken up into discrete tasks.

---

The UserTag system is nice - and we need it -- but it should work more like NodeTag:

https://github.com/publiclab/plots2/blob/master/app/models/user_tag.rb

https://github.com/publiclab/plots2/blob/master/app/models/node_tag.rb

NodeTag links Nodes to Tags. Tag stores tag names as strings, and each is unique (whereas NodeTag links a Tag to many Nodes).

That is, Tag has just arbitrary strings as its 'name' -- stored together, like key:value -- or for example activity:kite-mapping.

So we'd have to change a bunch of things, mostly in the User Tag controller:

https://github.com/publiclab/plots2/blob/master/app/controllers/user_tags_controller.rb#L23

And in the UserTag model (above). We can look at the tag controller a bit, although that's too complex and we need to simplify it:

https://github.com/publiclab/plots2/blob/master/app/controllers/tag_controller.rb

[jywarren]

This is complete in ; it now works with: #1494

UserTag.new(uid: user.id, value: "skill:#{value}")