Add help to to new CiviMail UI

In Civi-QuickForm, there's a standard help idiom (which involves a UI convention, an extra Smarty template, and some jQuery/AJAX – whose mechanics I haven't looked into). We need to come up with a visually-similar mechanism for including help on CiviMail's Angular-based screens.

(Note: I'm not sure if the Angular based screens should use/integrate-with/duplicate the help mechanism from Civi-QuickForm. Duplicate-implementation is a code-smell, but it may be simpler to do an Angular-style implementation on these screens, and I can't think of any use-case where the help data needs to be shared.)

Some example fields which could use help:

• Recipients / Dedupe
• Responses / Automated messages