Make layout more dynamic

[itrich]

At the moment, the template consists of an introduction section at top (if introduction is defined in the corresponding yml), a highlight section (if highlight is defined) and two substories (if substories is defined). This makes the newsletter layout very rigid.

We should at least allow to have either two or three substories (or 2x2 and 2+3 substories). The template for three substories can be generated with https://mosaico.io/.

[itrich]

Won't do as we'll maybe switch to another tooling with the new team member.