Test impact of macro typography settings on readability

This test has 4 variants:

1. **Control 1**: the current settings.
2. **Control 2 - No spacing**: consecutive paragraphs have no spacing between them.
3. **Variant 1 - No spacing, indents**: consecutive paragraphs have no spacing between them. A paragraph that follows another will have an indent.
4. **Variant 2 - More spacing**: line height and spacing between paragraphs are higher than the control.

It is set to run on 10% of our readers.