Add support for personal access tokens and service users

[pascal-fischer]

Is your feature request related to a problem? Please describe.
Being able to create personal access tokens to use for authenticating when sending API requests from 3rd-party applications or when using scripts.

Describe the solution you'd like
There should be an option to create personal access tokens for my user to use for authenticating agains the API. For better usability it would be good to have service users that are not bound to a specific person but can be created to hold tokens independently.