Refactor MultiInviter (#30500)

* MultiInviter: remove cancellation support

This is unused and untested, so we can basically assume it doesn't work.

* MultiInviter: factor out `handleUnknownProfileUsers` method

* MultiInviter: remove unused `ignoreProfile` arg from `inviteMore`

* MultiInviter: simplify `deferred` usage

No point in doing `deferred.resolve(this.completionStates)` everywhere

* MultiInviter.doInvite: do not `reject` for known fatal errors

Using `reject` for known, handled, fatal errors is somewhat confusing here,
since it looks like we swallow the error. (It's actually up to the caller to
check the recoreded `errors` and report them.)

Rather than rejecting, rely on the `_fatal` flag.

* MultiInviter: move finish logic to `.invite`

... for less `deferred` complication

* MultiInviter: rewrite loop as a `for` loop

Async functions are a thing in modern javascript, and way easier to grok than a
stack of promises.

src/RoomInvite.tsx

@@ -26,9 +26,9 @@ export interface IInviteResult {
 }
 
 /**
- * Invites multiple addresses to a room
- * Simpler interface to utils/MultiInviter but with
- * no option to cancel.
+ * Invites multiple addresses to a room.
+ *
+ * Simpler interface to {@link MultiInviter}.
  *
  * @param {string} roomId The ID of the room to invite to
  * @param {string[]} addresses Array of strings of addresses to invite. May be matrix IDs or 3pids.