graphqlbackend: add management console password fetch/clear functionality
Created by: slimsag
This will support informing site admins of the management console password and giving them the ability to copy it. For a logical outline of how this will work see https://github.com/sourcegraph/sourcegraph/issues/965#issuecomment-441552016
The actual UX which uses these GraphQL queries and mutations will be sent as part of https://github.com/sourcegraph/sourcegraph/pull/966 since it cannot be merged independently.
Helps #965