add sign up to navigation

[Oanh-and-only-Oanh]

Update the top navigation on https://www.topcoder.com/community/learn.

1. If the user is logged out, there should be Sign Up and Log in buttons
2. If the user clicks on SIGN ON, member should be routed to the account creation page.
3. the reURL should be https://accounts-auth0.topcoder.com/?retUrl=https://www.topcoder.com/start&mode=signUp

[Oanh-and-only-Oanh]

@Dara-K can you share design here.

[Dara-K]

@Oanh-and-only-Oanh Here is the design: https://marvelapp.com/prototype/86aaj33/screen/79554452

[luizrrodrigues]

@SathyaJayabal Deployed to QA and Beta env.

[SathyaJayabal]

@luizrrodrigues , verified on beta