Notifications: constrains for notif objects added and skipping null values of TemplateMessage

- no two TemplateMessages with the same templateId and locale can exist
- no two Regexes with the same regular expression can exist
- no two Receivers with the same target and locale can exist
- created new PerunException for each case in Notification module

- skipping undefined values in messages -> after merge this commit the pending messages on devel will be sent to Zora
 
- tested on my instance of Perun