GUI: added support for CHECKBOX widget on app form

 - allow to create CHECKBOX app form item type
 - value from checkbox widget is list of selected items 'values'
 - value can't be pre-filled yet
 - if item required, user must check at least one checkbox
 - fixed display of regex input form for mail
REGISTRAR: fixed null pointer and mail link

 - fixed null pointer when getting application by ID
 throw exception instead to display message to user in GUI
 - fixed '#' in URL of application detail for federation (use %23)
 in mail notifications