UI: Improve form consistency

Depends on !1953

See the commits for more details, the messages mostly speak for themselves.

# Highlights

## Tag form

Before:

![tag_before](/uploads/ee5ba6ef405749013f9e5717ebcf72f3/tag_before.png)

After:

![tag_after](/uploads/71bb543553190d91c03fd706cce35923/tag_after.png)


See merge request !1955