standardize on max_width or max-width

[benlk]

we use hyphens in some places and underscores in other places for max_width and min_width.

• [ ] change everything to underscores
  • [ ] widgets?
  • [ ] settings?
  • [ ] js?
  • [ ] blocks?
• [ ] migration as necessary for sites currently using dashes in places; depends on #79.