GitHub signup/login redirects to the same page without creating or logging in user

[courte]

Description
Certain users are unable to login or signup using GitHub. Upon clicking the login or signup button, the page simply reloads without displaying an error or a message. With further experimentation, facilitated by @circuitrider, it appears to have something to do with whether email addresses have been set to public in a user's profile.

In instances of this issue:

• GitHub authorization of the CodeMontage app is occurring for existing (but perhaps not signed in) users.
• A Service instance is not created for users with this issue.
• Neither is a User instance for new users.

Ask
Fix bug that is keeping new users from being created via GitHub.

To Do

• Confirm that errors created in /app/controllers/services_controller are showing in the template.
• Determine why GitHub email addresses must be public despite use of OAuth.

Additional Info
Click here for a video capture of this experience, helpfully shared by @jacquerie.

It's unclear how many users have had this experience. More information would definitely be helpful because this error is incredibly difficult to duplicate. If you are having this experience, please share the details in the comments below. Many thanks to @jarmstrng and @jacquerie for the initial report!

This issue is meant to consolidate #263.

[courte]

I've updated this issue with new information from further experimentation. Having a public email on GitHub appears to be Very Important to our OAuth process and is even included in existing error messages in ServicesController#create.

Unfortunately, those error messages aren't rendering due to flash issues, which should be fixed by #362.