Report invalid trace tags against the tags field

Report invalid trace tags against the tags field

Validate the tags as part of the trace and report any problems against
the tag string, so that the upload and edit forms show what is wrong
next to the tags field, and return a 422 status when either form is
shown again with errors.

This means removing the tag's validation of its trace, as validating the
trace from each tag cleared the trace's errors, so a trace with an
invalid tag followed by a valid one was considered valid. The tag still
requires a trace, as belongs_to associations are required by default.