MM-47288 refactor: convert channels/channel/ files to ts (#28483)

* refactor: convert channels/channel/ files to ts

- convert js files to typescript
- update types
- fix lint issues

Fixes: 21299

* fix: solve lint issues channels/channel

- fix lint issues
- udpate types

* fix: adjustment to test case MM-T1687

- code reorganization for test case MM-T1687

* fix: update types on function apiCreateCustomAdmin

- update types for apiCreateCustomAdmin on  user.d.ts so that
lint issue is solved

e2e-tests/cypress/tests/support/ui_commands.ts

@@ -638,7 +638,7 @@ declare global {
              * @example
              *  cy.uiPostMessageQuickly('Hello world')
              */
-            uiPostMessageQuickly(message: string): void;
+            uiPostMessageQuickly(message: string): ChainableT<void>;
 
             /**
              * Clicks on a visible emoji in the emoji picker.