Refine notification behaviour:

If you mark a post or comment as read even though you already marked the
corresponding notification as read, then you receive `null`.

backend/src/schema/resolvers/notifications.js

@@ -62,7 +62,7 @@ export default {
       let notification
       try {
         const cypher = `
-        MATCH (resource {id: $resourceId})-[notification:NOTIFIED]->(user:User {id:$id})
+        MATCH (resource {id: $resourceId})-[notification:NOTIFIED {read: FALSE}]->(user:User {id:$id})
         SET notification.read = TRUE
         RETURN resource, notification, user
         `

backend/src/schema/resolvers/notifications.spec.js

@@ -265,6 +265,20 @@ describe('given some notifications', () => {
               },
             })
           })
+
+          describe('but notification was already marked as read', () => {
+            beforeEach(async () => {
+              variables = {
+                ...variables,
+                id: 'p2',
+              }
+            })
+            it('returns null', async () => {
+              const response = await mutate({ mutation: markAsReadMutation, variables })
+              expect(response.data.markAsRead).toEqual(null)
+              expect(response.errors).toBeUndefined()
+            })
+          })
         })
 
         describe('on a comment', () => {