Add in configuration support for e-mail sending cases

[csnemeti]

In order to prevent spamming, admin should be able to configure when e-mails should be sent to users. Cases we need do decide:

• Issue created
• Issue assignment change
• Issue comment add/ delete / change
• Issue attachment add / delete / change
• Issue work log add / delete / change
• Project created
• Project status change: who should receive e-mail?
• User assignment change: who should receive e-mail?
• Project edit
• Project flow change
• User created: mandatory
• User password change OR reset: mandatory
• User e-mail change: mandatory
• Simple user edit: (name)
• User status change (Active <- -> Disabled)
• User role change

[csnemeti]

done but most of the cases are not tested yet..