Fixed user menu wideness
Fixed user menu wideness

The user menu div width was based on the width of it's parent, which
depended on the length of the user's preferred name. Thus, if a user had
a very long preferred name, the user menu could become very wide in a
silly way. This just sets a `max-width` on the div to avoid that.

closes #1338

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