Store "resigned" as an explicit reviewer state

Summary:
Fixes T11050. Today, when a user resigns, we just delete the record of them ever being a reviewer.

However, this means you have no way to say "I don't care about this and don't want to see it on my dashboard" if you are a member of any project or package reviewers.

Instead, store "resigned" as a distinct state from "not a reviewer", and treat it a little differently in the UI:

• On the bucketing screen, discard revisions any responsible user has resigned from.
• On the main /Dxxx page, show these users as resigned explicitly (we could just hide them, too, but I think this is good to start with).
• In the query, don't treat a "resigned" state as a real "reviewer" (this change happened earlier, in D17517).
• When resigning, write a "resigned" state instead of deleting the row.
• When editing a list of reviewers, I'm still treating this reviewer as a reviewer and not special casing it. I think that's sufficiently clear but we could tailor this behavior later.

Test Plan:

• Resigned from a revision.
• Saw "Resigned" in reviewers list.
• Saw revision disappear from my dashboard.
• Edited revision, saw user still appear as an editable reviewer. Saved revision, saw no weird side effects.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11050

Differential Revision: https://secure.phabricator.com/D17531