User management via API

[nhoening]

Also open user data / actions via REST API as far as possible (as Flask-Security allows, it might be fine that some things only work in our UI).

The reason is so that apps based on FlexMeasures can include some user management. If we

This is what I believe should be possible:

• List users (GET)
• Get a user (GET)
• Edit a user (PATCH)
• Deactivate a user (GET)
• Trigger password reset (GET)

Not possible through the API:

• Adding a user ― this is not possible in the UI either atm.
• Deleting a user (and their data) ― that should not be available through the API.
• Login
• Get an access token

Idea: Use Flask-Smorest this time, trying to understand if that works well (marshmallow integration and generating an open API documentation).

[create-issue-branch]

Branch issue-14-User_management_via_API created!