Remove duplication for CSS classes margin, padding, letter spacing between styles.css and responsive.css

[tdpauw]

css/styles.css and css/responsive.css have a similar set of CSS classes for margins, padding, letter spacing, background colour, ...

styles:

.letter-spacing-1
.no-margin {}
.padding-one {}
...

responsive:

.lg-margin-bottom-five {}
...
.md-display-none {}
...
.sm-no-margin {}
.sm-padding-top-six {}
.sm-display-none {}
...
.xs-letter-spacing-6 {}
.xs-no-margin {}
.xs-padding-seven {}
.xs-display-none {}
...

The ones defined in responsive.css should probably move to style.css .