ui: flex grid - give the ability / advice how to achieve this

[catlabs]

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report  
[x] Feature request
[ ] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead see https://github.com/NationalBankBelgium/stark/blob/master/CONTRIBUTING.md#got-a-question-or-problem

Current behavior

In old stark we have defined a set of classes to help end users to easily use flex layout

Expected behavior

We have different options:

• Reuse the same classes
• Tell the end user to use flex-layout (https://github.com/angular/flex-layout). This is more powerful as the first option, as they can also for instance add class according to the screen size (https://github.com/angular/flex-layout/wiki/Responsive-API). One of the problem with flex-layout is that it is only released in beta, but this is gonna change from Angular 7 (Pros & Cons using Flex-Layout angular/flex-layout#674)