Fix some issues where Conpherence would make to many draft requests

Summary:
A few minor fixes:

  - When we build a tag with `"meta" => null`, strip the attribute like we do for all other attributes. Previously, we would actually set the metadata to `null`. This happened with the Conpherence form.
  - Just respond to the draft request with an empty (but valid) response, instead of building a dialog.
  - `PhabricatorShapedRequest` is confusingly named and I should have caught this in review, but the basic shape of it is:
    - You make one object.
    - You call `trigger()` when stuff changes (e.g., a keystroke).
    - It manages making a small number of requests (e.g., one request after the user stops typing for a moment).
  - The way it was being used previously would incorrectly send a request for every keystroke.

I think I'm going to simplify `ShapedRequest` and merge it into some larger queue for T430.

Test Plan: Typed some text, no longer saw a flurry of requests. Reloaded page, still saw draft text.

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran, chad

Differential Revision: https://secure.phabricator.com/D8380