Migrate project categories model

[smcalilly]

Overview

This PR:

• Closes #153
• Closes #157

Notes

We'll need to change the data in staging and production once this deploys. The migration change won't upset the existing data with the category/subcategory fields — it'll just move it all the new ProjectCategory.name field.

Testing Instructions

• visit review app: https://ccfp-asset-d-feature-ca-yf8q7w.herokuapp.com
• to test the category model change:
  • visit the home page
  • make sure the projects list view works without a category
  • add a new project with the new categories
  • make sure the cip planner works without a category