Provide default UX for logging in when server responds 401

[timbl]

We have lots of login panels, but at the moment there are many cases in which the mashlib code puts a 401 error message into a status panel. It shouldn't really do that without giving the user a button to log in.

This needs to be coordinated with the 401 handling which the client or its successor does.

Possible solutions are for the developer to optionally pass in a place in the the DOM real estate which can be used for this. It could probably be used for 403 handling also, and other error displays.

[timbl]

Example user story

• User starts to use a solid-ui based app without having logged in
• The developer didn't put in UX code to require the user to log in
• The server rejects a fetch with a 401
• solid-ui sees there is provision for the login button, and so creates one of its own.
• the user is prompted to log in or sign up
• The user gets a first solid account and then the JUI collapses back to the login button
• The users presses the 'login' button and logs in
• The fetch() is retried and succeeds.
• The app continues where it left off