WIP: Port user tag to go

Fixes #3063 (closed) and #3064 (closed), except I just have "Delete" still because I got stuck on some side-quests.

Is blocked on !2804 (merged), for now I just rebased that series & put it at the start of this WIP merge request. Will rebase it out as this lands.

Still a few TODO etc. items in the commits, will give it a once-over at least once to investigate a couple of outstanding issues.

But in the meantime the "delete" works, and I'm opening the MR for initial review, CI etc.