This change adds the UI for status updates in the review request page. Status

updates are shown in two places:

If a status update is associated with a ChangeDescription, it will be shown

  in the entry for that change description.
Otherwise, it's assumed that the status update is for the initial diff (or any

  other attachments, etc. that were present when the review request was first

  published). These are shown in a new entry which always appears immediately

  below the review request box.

The one thing that's missing from this is the draft banners when replying to

any comments in the reviews associated with status updates. This is because

multiple reviews can be aggregated together into one entry, and we don't yet

have the ability to deal with that. Our proposed redesign for the draft banners

will solve this (since there will always be one banner at the top of the page).
If the reviews.status_updates feature is not enabled (which is currently the

default), any reviews associated with status updates will be displayed just

like all the other reviews, in their own entries.

Created a variety of status updates and checked that they rendered correctly.
Tested the default expand/collapse state for the new initial status updates

  entry (expanded only when there are no change descriptions or draft replies).
Disabled the feature in my settings and saw that all the reviews were now

  shown as regular review entries.