Add user settings API + Logic

[louneskmt]

This PR adds the routes and the logic to allow custom user settings.

New routes:

• GET /v1/account/settings: get existing user settings
• POST /v1/account/settings (request body: setting and value): update one setting to a new value, add it if needed

Related PR:

• https://github.com/getumbrel/umbrel-dashboard/pull/250

[bezysoftware]

This PR seems to be blocking couple of others, is there anything else that needs to be done?