MM-59854 Fully allow at mentions in message attachment field values and add E2E tests (#28018)

* Add descriptions to test cases for getNeededAtMentionedUsernames

* MM-59854 Load users who are at-mentioned in message attachment fields

* MM-59854 Add server support for at-mentioning users in message attachment fields

* Migrate support/external_commands from JS to TS

* MM-59584 Ensure that at-mentions never show in other message attachment fields

* MM-59584 Add E2E tests for how we load users based on at mentions

* Use new E2E test helpers in other places

* Update snapshots

e2e-tests/cypress/tests/integration/channels/websocket/handle_new_post_spec.js

@@ -102,17 +102,7 @@ describe('Handle new post', () => {
             const channel = response.data;
 
             // # And then invite the current user
-            cy.externalRequest({
-                user: admin,
-                baseUrl,
-                method: 'post',
-                path: `channels/${channel.id}/members`,
-                data: {
-                    user_id: user1.id,
-                },
-            }).then((addResponse) => {
-                expect(addResponse.status).to.equal(201);
-            });
+            cy.externalAddUserToChannel(user1.id, channel.id);
         });
 
         // * Verify that the channel is in the current user's sidebar and is unread with one mention
@@ -145,17 +135,7 @@ describe('Handle new post', () => {
             cy.delayRequestToRoutes([`channels/${channel.id}`], 100);
 
             // # And then invite the current user
-            cy.externalRequest({
-                user: admin,
-                baseUrl,
-                method: 'post',
-                path: `channels/${channel.id}/members`,
-                data: {
-                    user_id: user1.id,
-                },
-            }).then((addResponse) => {
-                expect(addResponse.status).to.equal(201);
-            });
+            cy.externalAddUserToChannel(user1.id, channel.id);
         });
 
         // * Verify that the channel is in the current user's sidebar and is unread with one mention