Sessions: ad-hoc session form takes a framed box above the list #262

Symptom. The ad-hoc session form sits in a collapsible framed box above the session list, pushing the list down and framing a form inside a page that already frames its sections.

Proposed fix. Move the form into a modal opened from a button on the right of the Sessions heading, drop the framed form box inside the dialog, primary button first, Cancel the same height as Connect, matching the other modals.

Fixed by ux/sessions-adhoc-modal. Part of the #254 series.