Fix duplicate key value violates unique constraint "userGroups_pkey"

This fixes the GraphQL error: insert into "userGroups" ("groupId", "userId") values ($1, $2) returning "groupId" - duplicate key value violates unique constraint "userGroups_pkey" when userGroups table was modified outside of web interface and there is already existing entry with ID that is the same of what is about to be added when assigning group to user.

server/graph/resolvers/group.js

@@ -71,8 +71,20 @@ module.exports = {
         throw new gql.GraphQLError('User is already assigned to group.')
       }
 
-      // Assign user to group
-      await grp.$relatedQuery('users').relate(usr.id)
+      // Check for unique ID conflict
+      const maxId = await WIKI.models.knex('userGroups').max('id as maxId').first()
+      const newId = maxId.maxId + 1
+      const idConflict = await WIKI.models.knex('userGroups').where({ id: newId }).first()
+      if (idConflict) {
+        throw new gql.GraphQLError('ID conflict detected while assigning user to group.')
+      }
+
+      // Assign user to group with unique ID
+      await WIKI.models.knex('userGroups').insert({
+        id: newId,
+        userId: args.userId,
+        groupId: args.groupId
+      })
 
       // Revoke tokens for this user
       WIKI.auth.revokeUserTokens({ id: usr.id, kind: 'u' })