Onwards container ab test

[abeddow91]

What does this change?

Adds a client side test for showing onwards journey containers to users.

Why?

So that we can measure the CTR on each onward journey container. By onwards content, we are referring to the 3 modules "top row", "bottom row", and "most viewed". Further information about what these 3 modules can contain can be found here

The test will have one control - the article as it currently exists with all 3 modules - and 3 variants, each with only one module visible. We are not currently concerned about the potential content variants within each row.

The test will run for 1 week.

It will run on all DCR articles with a sample size of 25% for each variant.

The corresponding frontend pr is here