Accept a wormhole code with text input and hand it off to the invite processing machinery

When a user wants the mobile app to gain read-only access to a folder they manage with the desktop app, they need to enter a wormhole code which will grant access to the configuration for that folder.

Give the user a way to enter the wormhole code as text.

This is a simpler step towards "pairing" functionality compared to the QR code-based approach and is useful in its own right for manual testing in a desktop browser during development where QR code scanning is less likely to be possible.