[native] Handle loading state when joining a community

Summary:
While the request is in progress, show an activity indicator and make the button disabled.

Depends on D7557

Test Plan: Introduce a sleep on server side. Clicking a button should replace its content with activity indicator. Size of the button shouldn't change.

Reviewers: bartek, kamil, inka

Reviewed By: bartek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D7558