Make following new projects easier

[njoyard]

Currently when we want to monitor new projects we have to connect to CircleCI as the peopledoc-tribe-js user and manually follow the project.

There is no endpoint on CircleCI api to list "followable" projects, but we could use the Github API (with a token from peopledoc-tribe-js) to fetch all existing projects and automatically follow them (using https://circleci.com/docs/api/#follow-a-new-project-on-circleci). Not all github projects are on CircleCI but we can either use the Github API to look for CircleCI hooks/CircleCI config file, or follow everything unconditionnally and cope with errors when CircleCI complains it does not know a project.