Add instructions to all the admin interface pages

[bcpierce00]

Every admin page should include very explicit instructions telling conference organizers what it does and how to use it.

In particular, the virtual rooms page should clearly explain the relation between the table at the top and the table at the bottom. (Maybe they should even be split onto two pages?)

There should also be a "Getting started" page at the top of the administrators tab, containing instructions for how to navigate the whole admin UI.

[EdNutting]

We should consider these ideas in the new UI.