General-purpose filterable "tags" for rats

[Rory-Reid]

While I want to build more things into the system as first-class citizens (and less around general-purpose very-flexible but not-that-useful stuff) it still feels like a degree of flexibility is allowed to account for niche use cases which I will not be putting into the app, and as a workaround in the interim for people that want to use the app and have a particular way of working that the app doesn't support yet.

To that end, having "Tags" for rats (not unlike github labels) would serve some useful purposes. Users could add data to a rat this way and filter on them specifically. Having them as a built-in customisable enum could eliminate typos and such.

This was brought up when discussing #26 as a means of adding "Borrow Back" to a buck. While I think that should be a specific feature, a tag system would allow breeders to do what they want before I build this.