List of contacts in Contribute panel

[althio]

Proposal: have a list of names for contact directly on the comment page in Contribute panel.
This list should be nicely formatted (see #300 and comments) so that:

• one can send directly a message: 1-click on name and link to http://www.openstreetmap.org/message/new/username
• one can copy/paste a list of people to be @-mentioned in the comment

This list (or rather these lists) could be generated as follow:

• everyone who worked on the tile
  • grouped for 1 copy/paste instead of browsing the list of locks/comments
  • useful to give feedback
• knowledgeable people: volunteers and active validators (could be retrieved automatically from the validated tiles of the current project?)
  • grouped for 1 copy/paste instead of browsing the other tiles for active people
  • useful to request help Add a "Ask a question about this task" button to the Contribute panel #360 Obvious "Help" button for every task #380
• project owner (also list project author on project detail page #371)
• others? admins?

Example:
at bottom of page in Contribute panel

• contributors

@user1
@user2
@user3
@user4

• volunteers and nearby validators

@user5
@user6
@user2

• project owner

@owner (OSM profile | private message)

Also... It could be clearer how the @-mention in the comment generate a message with private notification. I guess some people are not commenting because they do not think someone is going to see their comment.