Add Viewer to the API

[kevinhughes27]

part of #750

It is not currently possible to query data about the current user using GraphQl. This will be required by the new admin client to display the user's information and allow the user to update things like their name and email. Many GraphQL APIs have a viewer field that returns this information. It is also quite common to nest all the other query fields underneath viewer, I don't understand all the reasons behind this but I don't like it.

Task:

• Add a standalone viewer field to the GraphQL api
• Add some tests for this new field

Note - You have to run bundle exec rake schema_dump after any changes to the graphql api or else a test will fail. The admin_next client consumes this schema dump for relay which is why this is required.

[kevinhughes27]

@hiteshkanwar

[kevinhughes27]

@hiteshkanwar