Redirect for usage reports in legacy portal

[misterbisson]

The usage section of the legacy portal expects a year and month in the URL, like https://my.joyent.com/main/#!/usage/2018/4.

Right now, if you try to link to https://my.joyent.com/main/#!/usage it will redirect to https://my.joyent.com/main/#!/dashboard.

Ideally, it would redirect from https://my.joyent.com/main/#!/usage to https://my.joyent.com/main/#!/usage//

This is relevant to #24