Throw error if moderator tries to answer his own contribution in adminCreateContributionMessage.

backend/src/graphql/resolver/AdminResolver.ts

@@ -717,6 +717,9 @@ export class AdminResolver {
       if (!contribution) {
         throw new Error('Contribution not found')
       }
+      if (contribution.userId === user.id) {
+        throw new Error('Can not answer on own contribution')
+      }
       contributionMessage.contributionId = contributionId
       contributionMessage.createdAt = new Date()
       contributionMessage.message = message