add ability to know most recent registration date of a scheme in a DAO

Add a means of knowing, for each scheme currently registered to a given DAO, the datetime in UTC when the scheme has most recenty been registered (added or modified) to the DAO. This should work with both universal and non-universal controllers.

This is the minimum requirement. I can imagine someone wanting to know the entire RegisterScheme and UnRegisterScheme history of a scheme for a given DAO, but this is not necessary for this feature request. But if such a history is added, it should be possible to order it by block number (the datetime of (un)registration).

@A-Zak @jellegerbrandy

Start by tracing the RegisterScheme and UnRegisterScheme events for schemes registered to a DAO, including both universal and non-universal controllers. Done means the GraphQL data exposes the latest UTC registration datetime for every currently registered scheme; any optional history should be ordered by block number.

Written by the indexing model from the issue text.