Include JSON Content-type in client requests
(Still just test placeholders anyway)
Encode auth token from username and password
Store auth username and password with simple-prefs
Should use add-on API's passwords module instead, since it is designed specifically for managing credentials.
Use password manager to store/retrieve HTTP auth
Not sure this is really superior to using add-on prefs.
Revert "Use password manager to store/retrieve HTTP auth"

This reverts commit 90cefb0dec376d90ff7b3ed80fcb3250bb6bab64.
Store server address in prefs as well.