WebAlert: CSS cleanup

* Improves compatibility with Bootstrap 3.  (closes #1644)

Co-authored-by: Eirini Psallida <eirini.psallida@cern.ch>
Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>

Signed-off-by: Eirini Psallida <eirini.psallida@cern.ch>