[web] Disable AddKeyserverModal button while keyserver check is in progress

Summary:
We should never ever introduce an interface without considering loading state. This was a huge omission in the initial work on this project.

Depends on D12053

Test Plan:
Added a sleep for this video:

Reviewers: ginsu

Reviewed By: ginsu

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D12054