Extract validations

This is stuff that I was thinking we could pull out of the main platform gem. We only use this functionality in a couple apps, so it probably makes sense to just wrap it up in it's own gem that can be included when needed.

Here's what I have for the new gem: https://github.com/bellycard/belly_platform_string_validations

Does this make sense to you guys? Any thoughts?

@jdoconnor @craigulliott