Retreat: Add ability to reserve certain rooms for a retreat

[arborrow]

Certain retreats will use certain spaces (like the chapel or a particular room). It would be nice to be able to reserve these rooms and be able to see that they are not available for other events.

First we need a list of such rooms that will be used for this purpose.
Then we will need to create some UI modifications on the creation and editing of retreats to allow rooms to be reserved.