Refactor opportunities & experiments includes

[stoyan]

• Refactoring due to a recent global var mishap
• Add AssesmentRegistry singleton
• Categories are now constants
• Each opp calls register() to add an opportunity
• Each opp is now defined in an anonymous function (where needed) as to not bleed temporary vars as globals
• Using e.g. global $testStepResult makes it obvious where we have further space to improve, e.g. memoize the function that returns test step result and call it when needed, as opposed to rely on a global

Usage:

// get the single registry object
$reg = AssesmentRegistry::getInstance();

// register a new opportunity
$reg->register(
  AssesmentRegistry::Quick,  // category
  ['title' => 'etc...'], // actual opp
);

// use 'em all in common.inc
$assesments = $reg->getAll();

[scottjehl]

I'm pouring over this incredibly thorough and complete PR and I guess my only feedback so far is a minor variable edit change: assesment -> assessment?

This looks really great, @stoyan

[jefflembeck]

YES