Add ability to download/upload content migrations

[dturkington49]

For non-technical users, it would be useful to add the ability to not only create a content migration, but also download it through the browser and upload the content to a different environment. This enables non-technical users to not have to learn git, command line, or any deployment process. It could be as simple as:

1. Create the content entry in some sort of staging environment
2. Download/export the entry to a local file
3. Upload/import the entry to the production environment
4. And there was much rejoicing.