Specify repository URL for survey

[jarrah42]

Once the user authenticates, there needs to be a way to specify which repository to use for the survey. This could be in the form of a button or field, or some other UI element on the home page that is only visible once the user authenticates.

Just having a repo URL is not sufficient however. The user will also need to select a branch to use for the survey file, or possibly create a new branch. Also, in order to use a repository, the user needs read/write access to the repo. Even if they have this, I don't think it will be possible to write to a protected branch. I think it would be good to check this at the time the user specifies the URL so that write access can be determined. If write access is not available, then we will need to create a pull request containing the survey results.