Differentiate between a challenge group and a regular group

@atelomycterus - There's some user feedback we've gotten that the groups structure is a bit confusing for challenges vs. regular groups, especially since we're going to support the public round tables in the future, after the challenges are moved over.

What I think we should do:

1. Separate out "Challenge" from "Groups", so we'd have two separate sections under https://vanilla.topcoder-dev.com/groups/mine. They would both be groups, but we'll display "Challenges" at the top and "Groups" at the bottom.
2. Separate out "My Challenges" from "My Groups" on https://vanilla.topcoder-dev.com/groups
3. All breadcrumbs for challenges should have Challenges in the breadcrumb
4. All breadcrumbs for groups should have Groups in the breadcrumb

We'll need to support this in navigation (on the left nav, have separate Challenges and Groups), and in the REST API.

Final Submission Guidelines

Git issue link: https://github.com/topcoder-platform/forums/issues/133