MR Review Panel: Move Review bar to the top and make it always visible for Reviewers

Context

In the context of the wider effort to build an improved MR Review Panel as described in &14651 and #474794, this is step 1.

• Step 1: Review bar ⬅️ this issue
• Step 2: New drawer to preview Review pending comments (#525841 (closed))
• Step 3-?: Iterate on improving drawer

Tasks

• Move the existing Review bar from the bottom of the screen to the sticky bar at the top
• If user is a reviewer, show the Review bar even if no pending comments exist yet

Mockups

Review bar at the top	Content of dialog upon pressing Finish Review (unchanged)

Implementation Notes:

(To be confirmed by @mle)

• We'll iterate on the submission form in a separate issue
• Let's keep this set of changes behind a single feature flag
• This intermediate step will not be released on its own: keep the delete button and pending comments dropdown (previously drop-up?) as is.