Allow Project Owners to submit projects for code inventory file

We should consider creating a form on the code.mil website that allows project owners to submit details of their projects for the purposes of generating the `code.json` inventory file.

The steps toward this goal might involve…

- [x] breaking the existing `code.json` file into project-specific data files (e.g. `src/_data/code_json/move-mil.json` (or YAML…?) (#232)
- [x] add small Jekyll plugin to generate `code.json` file from data files (#232)
- [ ] determine a means of validating individual project data files and the resultant `code.json` file
- [ ] create the necessary pages on code.mil website to accept these submissions (this one's worth its own issue for sorting out design, implementation, etc.)
- [ ] form submission could/should create a pull request on this project's repository for evaluation and inform the user of the PR's URL (on say, a "thanks!" page)

Possibly useful Netlify documentation:

- https://www.netlify.com/docs/form-handling/
- https://www.netlify.com/docs/functions/