Add admin action to update slot names

* The slot object updates its name every time it is saved
* But sometimes its slotrooms are changed underneath it, and so the
name can become out of date
* This method is a simple way of updating the names for all the slots

List rooms in slot admin

In many parts of the schedule there are multiple slots with the same
start/end times, and it can be hard to find the one you want to edit.

Make this slightly simpler by listing the room names in the admin list.

Display (editably) the exclusvity of the slot in the admin

(Also add filtering so that we can easily find eg all plenaries which
are not yet flagged as exclusive)

Taking one out of PyCon's (US) book

We're lock-step with symposion, and upstream is dead.
Vendor it.