Issue title: Please enable HTTP/2 ( HTTP2 )
 Issue body: For some reason your Server dont provide HTTP/2. HTTP/2 is a great security benefit. It ensures that connections are faster and always encrypted. The always-encrypted is not by standard but thanks to mozilla and google to not implement the unencrypted mode.

Please enable HTTP/2
 Comments: 
 Comment 0: HTTP2 was enabled today after incompatible client version have been deactivated.
{'source': 'GithubIssues', 'repo_name_with_owner': 'tutao/tutanota', 'issue_id': 401334800, 'repo_stars': 4934.0}