Col-1073 Update GitHub auth

[matthuynh]

Changelog:

• Moved all OAuth variables (client ID and client secret) into .env files for both client and server
  • For client, make a .env file in client/.env, and add this line to it:
    REACT_APP_GITHUB_CLIENT_ID=<client-id-goes-here>
• Change path of localhost:3000/register2 to now lead to localhost:3000/register/authenticated. This is the URL that GitHub redirects to after the user authorizes the application
• Styling changes in Login and Register pages. Also added tooltips (eg. hovering over "email" field explains to user why we need their email)
• Minor refactoring

Testing:

• Paste the values of Client ID and Client Secret from here into you server/config/.env
• Paste JUST Client ID into client/.env
• Start the application, and see if you can still register

Extra Note:
Adding an .env file in the client still does not prevent users from digging out the client ID if they REALLY want to. From some quick Googling, it seems like the worst an attacker could do with our client ID is for phishing.

Closes #46
Closes #73

[jcserv]

Just gave this a go, looks great! The changes to registration are really good.