When selecting implicit column positions, actually create them

Summary:
Ref T4807. This is an alternative to D10179. The problem these diffs solve is that I want to be able to reorder a column's positions without having to load the actual objects, but that's difficutl because two positions may have the same sequence number (and I think it's good that we allow that, since it makes a bunch of other stuff way easier).

Instead of using the object ID (e.g., the task ID) to reorder positions with the same sequence, use the position itself. This is a little easier, is less ambiguous if columns eventually have several types of objects, and produces a better behavior when old objects are freshly added to a board. For example, if you tag T300 with #project, this new rule will push it to the top of "Backlog" while the old rule might have buried it deep. I think this behavior is desirable and more "natural".

When creating a group of new rows, we do order the batch by ID, so a group of freshly-tagged objects float to the top togehter in ID order. This seems like the most natural rule, too.

Test Plan:

• Loaded some boards with implicit objects on them (freshly tagged tasks) and saw rows create.
• Verified new rows created in the right order.
• Dragged some tasks around.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4807

Differential Revision: https://secure.phabricator.com/D10180