Flexbox helpers (#3047)

* Add flexbox helpers

* Add flex-{grow,shrink}: 0

* Order alphabetically

* Make classes shorter

* Rename to flexbox and used full attribute names

sass/helpers/_all.sass

@@ -2,6 +2,7 @@
 @charset "utf-8"
 
 @import "color.sass"
+@import "flexbox.sass"
 @import "float.sass"
 @import "other.sass"
 @import "overflow.sass"