management console UI should ask for password instead of using browser's built in basic auth form

Created by: slimsag

The basic auth form feels janky, we should just have the react app ask for the password when you visit the page instead. Then it would feel more integrated into the app, etc.

cc @francisschmaltz who suggested this + for designing how this form would look.

The form should only ask for the password (not username), and have a link to management console documentation (so the user can figure out where to get the password). No other information is needed on this page, but we could e.g. include the Sourcegraph logo etc.