we should offer subdomain routing in iodide

@hamilton and i were talking today about how we could quickly provide "nice" custom urls for iodide users, and he had the great suggestion of giving users the subdomain matching their username (i.e. bcolloran.iodide.io, bcolloran.iodide.mozilla.com). this could be extended to groups also (metrics.iodide.mozilla.com).

we could also let users set up custom slugs off of their subdomain that point at a specific notebook version, either as an additional subdomain prefix (bugzilla-report.teon.iodide.mozilla.com) or after the domain (teon.iodide.mozilla.com/bugzilla-report).

(i guess possible we could even build routing that supports both options and lets users choose whether they prefer a subdomain prefix or an after-the-slash slug?)

@teonbrooks @jtg567 -- would something like that be helpful?

@wlach any thoughts about this?