Add `faster-reviews` feature #3095

Joint issue/feature for #1069 and #883. Three parts in one feature:

1. Add "Review" button (https://github.com/user/repo/pull/1/files#submit-review) in sidebar (this also means un-hiding the Reviewers section in clean-sidebar)

   <img width="244" alt="" src="https://user-images.githubusercontent.com/1402241/82075974-65517f00-96dd-11ea-9e2c-808a926beda5.png">

2. Add data-hotkey="g r" attribute on both this button and on the native Review button in the Files tab

3. Autofocus the review field when it opens, something like:

    delegate('.js-dropdown-details > details', 'toggle', ({delegateTarget}) => {
        if (delegateTarget.open) {
            select('textarea', delegateTarget)!.focus();
        }
    }, true);