feature: setup without password / api-only client

# Issue details

### Duplicate?
Have you searched the issues of this repository if your issue is already known? yes

### Actual behaviour
when setting up the app, I have to provide username + password.
Apparently the app then creates an API client (with a generic name -- btw this could at least include a timestamp so to identify it on the website), and stores the credentials. I suppose this is then used instead of the original password to do all requests.

### Expected behaviour
If trust in the device is not that big (and/or just to minimize attack vectors), it would be useful to setup the app with extra login credentials, specific to this client.
If my assumption above is right, then allowing to operate with the API client ID+secret only, without username+password, would implement exactly this

# Environment details
- **wallabag app version**: 2.2.1
- **wallabag app installation source** F-Droid
- **Android OS version**: 9
- **Android ROM** LineageOS
- **wallabag server version**: 2.3.1
- **Do you have Two-Factor-Authentication enabled?**: no

# Your experience with wallabag Android app
Have you had any luck using wallabag Android app before?

Yes, works great!