Improve button loading states of MR reviews feature

Problem

The two buttons in the MR reviews feature Submit review and Add comment now have inconsistent loading states, as shown below.

1. When submitting a review, the “Add comment now” button loading state is unnecessary and confusing
2. When publishing a single review comment immediately, the “Submit review” button should be disabled

Proposal

1. When submitting a review, Simply disable all “Add comment now” buttons and remove their loading state
2. When submitting 1 review comment immediately, disable the “Submit review” buttons