Include strong in the list of valid html tags

The current list of html_valid_tags and html_valid_tags_single_line does
not include strong, but includes b . On the other hand, it includes both
i and em. For consistency, this should be adjusted. Also, rich text
editors have the tendency to produce markup which uses strong rather
than b, which is then escaped by MantisBT.

Fixes 0013062: Make strong a valid html tag by default