Create replacement template for all-auth signup

[nuwang]

If you attempt to signup with social auth, and a user account with that e-mail account already exists, an untemplated page is displayed to the user. This should be replaced with a properly themed page. See: #100 for context.

[thirdknife]

Will the template replacement apply to the UI project written in angular?

[nuwang]

Yes, hitting that django template url should probably just cause a redirect to an appropriate path in the Angular Single Page Application, where the new template will reside. As long as the path doesn't reside in: https://github.com/galaxyproject/cloudlaunch-ui/blob/master/proxy.conf.json, the SPA should get priority in handling it.