Fixed issue #27 - After user signs in - they should be redirected back

[CoderToCode]

commit for issue #27

[lakesare]

Good solution, @CoderToCode. Interesting use of the sessionStorage, I didn't know about this api.

One issue with this solution, is that if the user clicks the 'Sign In' button on the /courses page, - the user will be redirected to that /courses page instead of getting redirected to their /courses/learning page where they could see the welcome message.

I think we should only redirect user back to the:

• /courses/:id page (if they are on any urls related to this course)
• /users/:id page
• /courses/new page.