add a way to specify the the url path under which all goat-guardian endpoints live

[cdepillabout]

Currently, goat-guardian is assuming that the Twitter OAuth login and callback endpoints are at /twitter/login and /twitter/callback.

However, it would be great to allow the operator to be able to specify a parent path.

For instance, if you run goat-guardian and specify the environment variable GG_URL_PATH as /auth, then goat-guardian would assume the Twitter OAuth login and callback endpoints are at /auth/twitter/login and /auth/twitter/callback.

This would need to work for any other authentication endpoints as defined in #1 or #2.

[arowM]

This could be "after MVP" milestone.

The reason is as follows.

• Implementing this feature does not seem to inject any vulnerabilities
• End users have workaround to change their upstream application paths (though it's not the best solution)

[cdepillabout]

Okay, let's do that! I'll change it to "after MVP"!