System-wide settings

[sheepman4267]

In OpenShow, there are a good few things that are currently jammed into the django settings module which don't belong there, plus a further category of hard-coded stuff that should be configurable. This demands some UI for system-wide settings.

My implementation idea for this is to have a Django model with fields for all the settings, plus a "Current" field with constraints so only one instance can be true at once. This would allow for settings rollbacks and versioning.