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