Apply GenericRelations to Tag model [15 Nov, 23.59]

[cerob]

GenericRelations are something cool that we are using for our database.

Since we changed user interests to tags, it's a great idea to implement this feature for user tags too.

[mertakozcan]

This will also be useful for recommendation stuff. For example, starting from a tag, we can query all items that are tagged with that tag. 😀

[mertakozcan]

Do we need an API for this? I can't remember what we've talked. @cerob

[mertakozcan]

Since we won't need an API for this, I think it is unnecessary to make a GenericModel for Tag. However, I created a mixin anyway. (i.e. if some Tag operations will be done on Event, it'll be done over Edit Event endpoint)