Show a preview of the tags when uploading or editing a trace

Show a preview of the tags when uploading or editing a trace

The help for the tags field says that they are comma delimited, but
if there aren't any commas they are split on spaces instead, so
"Foo Bar" gives two tags rather than one.

Rather than trying to explain that in the help, show the tags that
will be created after it as they are typed, split in the same way as
Trace#tagstring= does, and change the help to say whether they are
comma or space delimited. The help is left as it was unless there
would be more than one tag.