Make it easier to use custom tools in BG cloud instances

[heralden]

This is easy when BG is not running on the cloud as filesystem access is trivial and you just pop the tool into the folder. For cloud instances, we might want to support using custom tools without having to require them to be published to NPM. Using git might be a good idea - the admin would paste in the git URL on the BG tool store page to install it. This would make it very easy to fork an existing tool, make small changes, then install it.