Multitenancy

Let's put `site_name` in metadata and init the site on hook. This would help in multi tenant installations sharing the same mandrill account creds (in `common_site_config.json`)

Start by reading common_site_config.json and tracing how site metadata and initialization hooks are handled in this repository. Confirm how site_name should be stored and when the Mandrill site should be initialized for shared credentials; done means multitenant installations can use separate site identities without changing the shared credentials.

Written by the indexing model from the issue text.