Encapsulate flex-box properties with scss mixins

It looks like cross-browser prefixes is applied inconsistently throughout the project. Search for flex:, and you'll see that this is sometimes used by itself, and sometimes accompanied by -webkit-flex:. Rather than removing these cross-browser prefixes altogether, let's encapsulate their usage in some mixins:

https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Flexible_Box_Layout/Mixins

TODO

• Create a place to house the flex-box mixins
• Generate a list of files that would need to be updated