Reintroduce /users endpoint

[vivienlacourba]

This is a follow up of #46 (comment)

Recently we harmonized our configurations and now all W3C public groups (ie WG, IG, CG, BG) have their list of participants public.

Besides groups participants the following list of users are publicly available on the W3C website:

• TR Editors
• Team
• Alumni

It would be handy to reintroduce the /users endpoint.

I am checking internally to see if the W3C Privacy policy allows us to list anyone with a W3C account or if we should only include to the above users.

[sandhawke]

+1 this would be really nice