Implement optional login

[redshiftzero]

Requirements:

• Users should be able to open and use the SecureDrop client application without authenticating to the server, i.e. when disconnected from the network entirely. In this scenario, they are unable to sync, and unable to reply to sources, and the corresponding UI elements should be greyed out / inactive.

• When requesting or sending new content to the server, they'll need to get an API token, by providing their username, password, and 2FA token. We need a UI form that allows them to do this. The actual login functionality would be provided by the SDK's API.authenticate method.

Related: a common complaint from our journalist user research is that there are too many different logins, passphrases, etc., so if we can ease the burden, then that would be good from the user's perspective. Recall in order to get to the SecureDrop client application, they need to also know both the login for the Qubes user, and the FDE passphrase for the workstation (these can be the same).

[ntoll]

This was fixed by #29