Add a modal dialog to let people know they're in Via

[seanh]

Similar to:

• Add a banner to let people know they are inside Via

Add a modal dialog that the user has to click through before they get to see the proxied page.

The dialog could be added only the first time the user uses Via, once they've dismissed it once this dismissal could be saved (e.g. in a cookie or localStorage) so that we don't show the dialog again.

Or maybe we'd remember the dialog dismissal only for logged-in users.