Change form submit button's label when signing in/up to audio.com

### Problem to be solved

When signing in, the button label changes to "Loading..." whereas when signing up, it remains "Create account";

### Your idea

When signing in/up, the button label changes to "Loading..." in both cases.

### Prior art

No files or tests are named in the issue. Start by locating the sign-in and sign-up form implementations in the Audacity desktop UI, compare their submitting-state labels, and verify that both display "Loading..." while authentication is in progress.

Written by the indexing model from the issue text.