implement verification for additional mails

- mails added by (sub)admins are automatically verified
- provisioning_api controller as verification endpoint
- IAccountProperty gets a locallyVerified property
- IPropertyCollection gets a method to fetch an IAccountProperty by value
  - an remove equivalent was already present
- AccountManager always initiates mail verification on update if necessary
- add core success template for arbitrary title and message

Signed-off-by: Arthur Schiwon <blizzz@arthur-schiwon.de>