add "promoting"

[frnsys]

articles, events, and stories (not concepts) can be "promoted" which is basically just an indication that a user thinks other users should see this.

requires:

• a promoters attribute added to articles, events, stories, which are the users which have promoted the entity
• api endpoints that allow users to POST (create) or DELETE promotions for a particular entity (i.e. adds or removes the user from the entity's promoters attribute, respectively)