Add priority group headers to workboard columns (display only)

Summary:
Ref T10333. When workboards are ordered (for example, by priority), add headers to the various groups. Major goals are:

• Allow users to drag-and-drop to set values that no cards currently have: for example, you can change a card priority to "normal" by dragging it under the "normal" header, even if no other cards in the column are currently "Normal".
• Make future orderings more useful, particularly "order by assignee". We don't really have room to put the username on every card and it would create a fair amount of clutter, but we can put usernames in these headers and then reference them with just the profile picture. This also allows you to assign to users who are not currently assigned anything in a given column.
• Make the drag-and-drop behavior more obvious by showing what it will do more clearly (see T8135).
• Make things a little easier to scan in general: because space on cards is limited, some information isn't conveyed very clearly (for example, priority information is currently conveyed only through color, which can be hard to pick out visually and is probably not functional for users who need vision accommodations).
• Maybe do "swimlanes": this is pretty much a "swimlanes" UI if we add whitespace at the bottom of each group so that the headers line up across all the columns (e.g., "Normal" is at the same y-axis position in every column as you scroll down the page). Not sold on this being useful, but it's just a UI adjustment if we do want to try it.

They aren't yet recognized as targets by the drag list UI, so you can't drag cards into an empty group. I'll tackle that in a followup.

Test Plan:

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T10333

Differential Revision: https://secure.phabricator.com/D20247