[WIP/PoC] Multi-user support (backend only)

[WithoutPants]

This is a WIP implementation of multi-user support (#12).

Currently stores user information in the config file. I expect to change this to store user information in the database before this can be considered ready to merge.

Adds fairly coarse-grained roles: READ, MODIFY and ADMIN.

Requires that the first user created have the ADMIN role. Backwards compatible with the existing username/password config fields - which assume an ADMIN role.

Tested briefly by creating some users and logging into a READ user. Attempts to modify objects currently gives an unauthorized error, which should be improved to give a more verbose error.

Expected next steps:

• determine final role types
• add a schema migration and create a user store implementation
• add user-specific api-keys

Potential future work:

• UI support
• Plugin permissions
• Adapt UI for various role types
• User-specific object ratings
• User-specific preferences
• Content permissions

[Gykes]

Commenting to keep track so I can do frontend work

[sleetx]

Feature request / discussion: #2337