Make use of Bootstrap breakpoint mixins

[hoetmaaiers]

We should replace most css defined breakpoints with the bootstrap provided breakpoints: https://getbootstrap.com/docs/4.0/layout/overview/. This way it is possible to update the bootstrap configuration and the breakpoints will update accordingly.

[12rambau]

can someone double check, but I think we are now only using mixin breakpoints for the few elements that needs it (sidebars) the rest is handled by flex elements

[choldgraf]

I think that we can now close this as we're using the bootstrap breakpoints mixins in general