Misc. updates to the config rendering process

[jackmakesthings]

A few changes I have been making on projects lately that could be backported to gesso:

• WP has built-in fluid sizing and we have a map for responsive font-sizes - these can work together with adjustments to the addToFontSizes function in renderJson.js
• change the constraint mapping in the same file so sm => content & md => wide content (currently it's md and lg respectively); better reflects how our designs & the WP container system relate to each other