Add site discovery behavior

[zstumgoren]

A site contains a range of governmental bodies and a history of meetings that vary by entity.

We should update base.Site to add a discover method (not required to be implemented) that can be implemented in CivicPlus and other subclasses. The method should profile a site and return as much of the following information as possible:

• committee name
• earliest know meeting date
• entity ID, if available (e.g. in CivPlus, this would be CID, which in turn can be used to craft targeted searches in CivicPlusSite.scrape or some other future public method