Fixes:
https://linear.app/zed-industries/issue/Z-2416/improvements-to-feedback-submission

We get a lot of duplicate messages through our in-app feedback. My best
guess is that because we do not tell the user we are doing anything, and
because submission takes awhile, users are hitting the submission button
mutliple times. This PR blocks the submission code, once an initial
submission is sent. If the original submission fails, we unblock the
submission code. The submit button is disabled and enabled accordingly
as well.

Release Notes:

- N/A