Bug_Submitter: remove obsolete dummy text

The explicit error checking now requires the user to fill in the summary
and description, so there can be no chance of submitting a bug report
without these fields populated (at least superficially).
Bug_Submitter: visually flag invalid fields

This is hopefully clearer than just complaining about the first invalid
field, since it flags *all* invalid fields, and the flags do not
disappear until the user at least focuses each invalid field.