Tenant management UI

This PR is an initial implementation of #450 and includes:
 * new "tenants" tab in nav for admins, re-using the services icon for now
 * tenant list with workers/queues display (does not live-update yet)
 * tenant delete button
 * tenant create/edit form

and as a bonus:
 * CSS tweak for the navbar on small screens

This is fine with me, but @nitinmotgi may have something to say about it being first.