search: add graphQL schema and stubs for code monitoring

Created by: stefanhengl

This PR adds code-monitors to our GraphQL schema. The goal is to first merge a stub implementation to unlock frontend work.

The schema follows this design. There are still some things missing (events, mutations, mock-store), but I wanted to break it down to limit the size of the PRs.

For the review, I think the most important parts are the changes to the schema.graphql and user.go.

Here is an example of a query that will work:

{
  user(username: "stefan") {
    monitors(first: 1) {
      nodes {
        id
        createdBy {
          username
        }
        createdAt
        description
        owner {
          __typename
          ... on User {
            username
          }
        }
        trigger {
          __typename
          ... on MonitorQuery {
            query
          }
        }
        actions(first: 1) {
          nodes {
            ... on MonitorEmail {
              recipient {
                __typename
                ... on User {
                  username
                }
              }
              priority
              enabled
              header
            }
          }
          totalCount
        }
      }
      totalCount
    }
  }
}