Change log out authentication

[Stahl99]

Description:
Change the authentication when deleting an API key at log out from basic auth to authentication via api key. After this change the password no longer has to be stored locally.

Requiremnt:
The server has adapted it's authentication mechanism.

DoD:

• The authentication for the deletion is no longer done via basic auth
• The password variable is no longer held in the program memory and the preferences
  - [ ] Adapt the onErrorResponse function
  => not possible due to server response