Make session interest reversible & visually consistent on session page

[pcantrell]

Currently, the session page has a “Yes, I’m interested!” button that disappears when clicks: it is possible to vote for a session from that page, but not unvote it. Elsewhere in the UI, session votes shows up with the MinneStar star, and can be toggled on and off.

We should make the session page consistent with the rest of the app:

• Make it possible to vote and unvote a session from the session page.
• Use the Minnestar star to indicate a session vote in a way that is visually consistent with the rest of the app.

Probably the way to do that is to create a checkbox styled to use the same circle / star toggle appearance as the session listing and schedule pages, but other design approaches could work too.