Review requests and actionable comments

My massive change is now ready for beta testing and review. Feel free to add comments and tweak stuff.

This change keeps track of review requests and "actionable" comments. Now you can see all the actions you need to take on one screen.

When someone requests that you review their commit, the request will show up in your "Code review requests" list.

When a reviewer adds a comment to a code review, it will show up on the commit author's "Actionable comments" list as "New". When the commit author resolves the comment (by clicking "Resolve" on the comment), then the comment will show up on the reviewer's "Actionable comments" list as "Resolved". The reviewer can then check that the comment was resolved to his satisfaction and delete it from his "Actionable comments" list or reopen it (by clicking the "Reopen" button on the comment).

Only comments marked as "Response requested" will show up in lists. (This naturally excludes all existing comments.) New comments will have the "Response requested" checkbox selected (unless it is a reply to another comment).

All the new lists are under a new major tab called "Reviews" at the top of the screen.

There are also other lists that are less commonly used and don't require action, that are collapsed by default. Those lists show: "Recently completed requests", "Code review requests (from me)", "Pending comments", and "Closed comments".