admin: Add ability to assign talks to Tracks in All Submitted Talks page

[jwboyer]

Ideally, the talk selection panel would be able to sort through the submitted talks on the fly even as registration was open. It would be beneficial to be able to slot them into particular Tracks while submissions come in.

Tags could be used for this, but there are a couple of issues with Tags. The first is that they're optional and freeform. With no enforced convention, it is difficult to rely on the data that a submitter enters. The second issue is user editing of the Tags later. They may inadvertently remove a Tag that an admin adds, etc.