Add "Practice" to drop down navigation

[Oanh-and-only-Oanh]

Add "Practice" to drop down navigation next to Gig Work

• Page is visible to all users (members and non-members). But only registered members can register for the practice challenges and compete
• When a user clicks on "Practice", direct user to new URL: https://www.topcoder.com/community/practice
• https://www.topcoder.com/community/practice URL is a new Practice page (in development)

Deadline to go LIVE: May 13, 2020

[luizrrodrigues]

@Oanh-and-only-Oanh Can you confirm if URL is right, looks like not present in Dev:
https://www.topcoder-dev.com/community/practice

[luizrrodrigues]

The navigation is ready in nav-practice branch:
#4364

On ~HOLD until we have page ready too.