Prevent caching of Ajax requests for the events queue

Problem/Motivation

We came across a number of occasions where events were re-triggered by the user using the browser to go back a page or refresh a page.

Steps to reproduce

One example is:

1. Visit a product category page and select a product
2. Add the product to the cart
3. Press the browser back button - still the product page
4. Press the browser - the product category appears and both visit product page and add to cart events get resent

Proposed resolution

Edit the EventsController::getEvents method called by Ajax calls to set the cache control header to include no-store.

Remaining tasks

More testing and if approved commit (I have supplied a proposed patch)

User interface changes

None

API changes

None

Data model changes

None