Make sunlight_announcements functionality more generic

[jpmckinney]

• Alias organization_announcements to sunlight_announcements in Mongoid, i.e. the field name in MongoDB is still sunlight_announcements to avoid breaking existing databases, but the field name everywhere else in the code is now organization_announcements.
• Add organization section to config.yml, with name and url keys. To upgrade, simply add this section to your production config.yml file. I haven't used these new configuration variables everywhere - I've just updated the parts related to the announcements functionality.

[konklone]

Good call, nice comprehensive coverage. This doesn't touch the actual alert emails that get sent out though - especially their footers. But there's a lot more org-specific stuff there anyway, more than a name and URL can handle. Thanks for dong this.