Support github and updated boilerplate

[marcoscaceres]

People that don't use mailing lists (that's me!) should be able to list a GH repo as the primary way to provide feedback about a spec.

As @AFBarstow proposes:

The boiler plate asking for feedback via GH and the Status section
asking for feedback via p-webapps is certainly suboptimal. AFAIK,
PubRules mandates the comment list. If we can't get that restriction
lifted, I recommend the relevant text in the SotD clearly state using GH
for comments/bugs/issues is strongly preferred and the list should
only be used as a last resort.

[plehegar]

For reference: https://github.com/w3c/specberus/blob/master/lib/rules/sotd/mailing-list.js

[marcoscaceres]

Mailing lists are no longer required by the w3c + we now have github option.