Suggest mixins

[vadym1930]

• button
• container
• fullwidth-bg-color
• full-width-container
• icons (before, after)
• mq (media-query)
• grid-layout
• flex-layout
• image-crop-height
• input-placeholder
• rem-calc
• background-center

[serhii-checheniev]

As I can see we have all the mixins in our old draft theme https://github.com/lemberg/draft-theme
But we have @mixin grid-layout here https://github.com/lemberg/draft-theme/blob/1.x.x/STARTER/scss/framework/_layout.scss that is not about grid, it's about flex-layout. So the name is incorrect. And then the real grid-layout should be added.