Generalize rules for "workflow" sigil

Summary:
See discussion in D6130. Basically, all of these should activate workflow:

  <a data-sigil="workflow" href="...">...</a>

  <div data-sigil="workflow">
    <a href="...">...</a>
  </div>

  <form data-sigil="workflow" action="...">...</form>

  <div data-sigil="workflow">
    <form action="...">...</form>
  </div>

The only case where we don't want to activate workflow is this one:

  <form data-sigil="workflow">
    <a href="...">...</a>
  </form>

Here, the form should workflow but the `<a />` should not.

These cases aren't really covered:

  // Undefined no matter where "workflow" is because it's nonsense.
  <a><a>...</a></a>

  // As above except like a million times more dumb.
  <form><form>...</form></form>

  // This one is ambiguous. The <a /> will currently workflow. We don't do
  // this anywhere and probably never will. If we want a different rule we
  // can cross that bridge when we come to it.
  <div data-sigil="workflow">
    <form action="...">
      <a href="...">...</a>
    </form>
  </div>

Test Plan: Clicked/submitted some things with workflow.

Reviewers: chad

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D6131