Make collectives and campaigns listing page

[berleant]

See discussion here: https://github.com/debtcollective/parent/issues/155#issuecomment-396441673

Acceptance criteria

• [x] /collectives routes to a list of collectives
  • [x] add custom field (discourse-seed)
• [x] /campaigns routes to a list of campaigns

[berleant]

https://github.com/debtcollective/discourse-custom-pages/pull/1