More customizable Heartbeat prompt

[rehandalal]

Copied from: mozilla/normandy#331

Original bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1294504

S&I wants to expand the capabilities of the show-heartbeat action:

• The icon should be customizable from a set of defaults.
• The engagement button option should allow for multiple buttons, each with separate URLs and thank-you messages.
• There's interest in adding different ways for users to respond besides buttons and stars, such as Likert scales.

See original issue for full discussion.