Add download count from GitHub to the project_github module

[jenlampton]

After @serundeputy worked on backdrop-ops/backdropcms.org#284 @quicksketch asked that work be contributed upstream into the project_github module.

• A property called project_github_downloads should be added to project nodes.
• It should not be editable on the node edit form, similar to the project_name property.
• It should be positionable via manage display, similar toProject usage.
• All the magic from borg_project_metrics.module should be moved into project_github.module
• Add views integration for the download counts so we can show it on the modules page.