Automatically generate visibility classes

Replaces most of the visibility classes with a mixin that simplifies
the process of adding or removing breakpoints and their associated
visibility classes. Resulting CSS is comprehensive without containing
any extraneous or nonsensical classes, and is far less error-prone.
Made style adjustments to new visibility functions
Separates legacy visibility classes in dynamic visibility class generator loop

Removes functionality to omit visibility classes based on relative
size, in favor of generating consistent classes. Separates legacy
visibility so they can be omitted.
Isolates legacy visibility classes to legacy breakpoints only

The .show/hide-for-[size] and .show/hide-for-[size]-down classes are
now only generated for small, medium, and large breakpoints, and only
if legacy visibility classes are enabled.
Resolved conflicts
Added visibility class html variables to settings file.
Merge pull request #4422 from zurb/alandaguegreene-alandaguegreene-dynamic-visibility-classes

Added alandaguegreene's dynamic visibility classes