Fixed /learner -> /learner/username redirect
Fixed /learner -> /learner/username redirect

I accidentally broke this functionality when I merged
fb75ab9769384538075e99e693c64b90a1d2a6cd, due to the change from a react
component react-router setup to a plain object one. This fixes it!

closes #2415
pr #2416

mitodl/micromasters,mitodl/micromasters,mitodl/micromasters,mitodl/micromasters