Implement GraphQL mutations

This is a very broad area, which is going to involve:

• Some changes in <SearchAndSort> itself – hopefully only in the ConnectedSource classes – which are the direct subject of this issue.
• Related changes to make use of these in the various apps' new/edit forms components – <UserForm>, <InstanceForm>, etc. This will be its own issue, similar in scope to STRIPES-515.
• Support for the mutations on the server side, in mod-graphql. I believe Jason already did the necessary proof-of-concept work, but similar things will need to be done in the schema/resolvers for each app, so this may be the point where we start trying to find good way of automating this configuration and code. It will certainly require at least one new issue, probably several.