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/autocomplete/helpers.ts

@@ -243,13 +243,8 @@ function createChannel(channelType: string, teamId: string, userToAdd: Cypress.U
         if (userToAdd) {
             // # Get user profile by email
             return cy.apiGetUserByEmail(userToAdd.email).then(({user}) => {
-                // # Add user to team
-                cy.externalRequest({
-                    user: getAdminAccount(),
-                    method: 'post',
-                    path: `channels/${channel.id}/members`,
-                    data: {user_id: user.id},
-                }).then(() => {
+                // # Add user to channel
+                cy.externalAddUserToChannel(user.id, channel.id).then(() => {
                     // # Explicitly wait to give some time to index before searching
                     cy.wait(TIMEOUTS.TWO_SEC);
                     return cy.wrap(channel);