Show user a notice when they view "My Projects" for feed but haven't joined any projects

Summary: Currently, we show them everything. Instead, show them an explicit notice.

Test Plan: Looked at "My Projects" feed with no projects.

Reviewers: btrahan, vrana, jungejason

Reviewed By: jungejason

CC: aran

Maniphest Tasks: T1015

Differential Revision: https://secure.phabricator.com/D2143