Improve Mobilizon instance UX

[privatemaker]

Currently the extension offers no "login" button near the Mobilizon credentials. At a minimum it should:

• Have a login button
• Test authorization
• Display success/failure message when logging in
• Hide authorization fields once successfully auth'd

Going a step further, the extension could incorporate a list of know + reliable Mobilizon instances and offer these in a dropdown menu while also allowing for custom input field.