Impersonation E-Mail Notification

**Is your feature request related to a problem? Please describe.**
Impersonation of other users can have serious consequences, including in third party systems where the user is authenticated. To improve transparancy, impersonated users should know that they was impersonated.

**Describe the solution you'd like**

Send an E-Mail to the user which was impersonated, including:
- The user who impersonated him
- The reason why the account was impersonated

This should be an opt in on system level.

**Describe alternatives you've considered**
Deactivating impersonation feature