Save modules directly to GitHub

[Dylan Hall]

Now that we have the ability to pull modules from GitHub, the next feature we'll want is to be able to push updated modules directly to GitHub.

This will require some form of authentication, which should be optional (people should still be able to use the module builder anonymously), as well as the ability to choose alternative repositories (most people do not have access to the main synthea repo, and would want to be able to push to their fork).

Also consider some form of version protection - if 2 people try to edit one file at the same time, one should not be able to overwrite the other without realizing it.