forms: Allow decl of promt and default for choices

Allow the administrator to enter a prompt to be shown in the choices
drop-down for choice fields. Also add the option of selecting a default
value from the list initially rather than displaying a prompt.
Merge pull request #427 from greezybacon/feature/choice-field-prompt

forms: Allow decl of prompt and default for choices

Reviewed-By: Peter Rotich <peter@osticket.com>