add admin page

In order to allow users to configure Karapulse (configuring Spotify, etc) we'll need a password protected admin web page. I was thinking of something like this:

Server

• add /api/admin/auth POST endpoint, taking the admin password as input and returning a JWT on success
• add /api/admin/config GET endpoint, returning a JSON with the current configuration
• add /api/admin/config POST endpoint, taking a JSON to update the configuration
• /api/admin/config requires a valid JWT except if there is no password defined yet, then only setting this password is allowed (required for initial setup)
• Redirect any HTML request to the admin page if no admin password is defined, forcing user to define one before starting to use Karapulse.

Frontend

• Add an admin button, asking for password and requesting a JWT if none stored yet
• Have this admin page request the config and displaying UI to change it.
• Store the JWT in local storage or cookie so it can be re-used.