Multi choice proposal tags option

Currently "category:tag_name" tags have to be chosen between (exactly one choice must be supplied). It would be nice to also add other configurations such as "1 must be supplied, but multiple choices can be supplied" and perhaps also "multiple choices can be supplied, but it is not mandatory". We could consider a few different designs for this:

1. Add them as category::tag_name and category:::tag_name, and make use of #493 (closed) to provide a clear UI.
2. Redesign the system to be more complex and not use the same tags, make separate models (lots of extra effort?)