Screen reader classes

[Joozty]

We need to implement css classes for screen readers, i.e. only display content to screen readers.

Inspiration can be taken from here:

https://github.com/twbs/bootstrap/blob/master/scss/mixins/_screen-reader.scss

[viktor-yakubiv]

I will do it here for the time being and move outputs at some point back. Thank you!

[viktor-yakubiv]

Partially fixed by #38