Split Herald rule creation across several steps

Summary:
Ref T4264. Currently, you choose a rule's content type (revision, commit, hook) and rule type (global, personal) on the same screen.

• I want to make some rule types unavailable for some content types (e.g., personal hooks make little sense).
• I want to make content type selection use a radio control instead of a dropdown, so it can explain what the content types do in more detail.
• For new "object" hooks, I want to add a third step where you'll pick an object to bind to.

Split rule creation out into two steps. I think this won't get complicated enough for PHUIPagedFormView, but maybe I'll swap it in if this gets messier than I think.

Test Plan: Created some Herald rules, used back/cancel/etc.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4264

Differential Revision: https://secure.phabricator.com/D7845