Don't save trace tags until the trace is saved

Don't save trace tags until the trace is saved

Assigning a new set of tags to an existing trace saved them immediately,
so an invalid tag raised ActiveRecord::RecordNotSaved instead of failing
validation. The API returned a 500 error and the edit form showed an
error page.

Build the new tags instead so they are validated and saved along with
the trace, and update the trace through the API in a transaction so the
old tags are kept if the update fails.