CatalogSync should keep track of historical enrollment data for sections

[haydenmc]

This would be an expensive feature, but really useful. Imagine graphs showing when classes tend to fill up after registration opens (and when people tend to drop after the term starts...).

It would involve splitting enrollment numbers out into a separate relational table, and maybe providing fields for 'current' enrollment figures (so as not to cause performance issues when not querying for historical data).

Care should be taken not to regress performance too badly with the addition of more relational data... at what interval should we record historical enrollment figures?