External ids for organizations

[jace]

External ids should be linkable to organizations instead of users. This is similar to login providers, but cannot be used for login and links to GitHub organizations, Facebook pages, etc. Twitter ids behave the same as with a login provider, while Google is not available (except G+ pages).

[miteshashar]

External ids should be linkable to organizations instead of users.
Better to say External ids should be linkable to organizations OR users.?
Also, this would be mutually exhaustive?

[jace]

This will most likely have to be a separate table since the data is slightly different (for eg GitHub organizations instead of users, but connected via a user's external id).

[jace]

Deprecated with #232. This should be handled in apps.