config schema endpoint

Create a config user schema endpoint which will allow a user to add fields to the schema
- first version: write back into the schema.json file the new fields added, the user can then commit the changes and create a PR. Someone can review the changes and accept the new fields, the PR is merged and the production application (heroku for now) is reloaded with the new schema
- second version: use the Github api to create the commit and PR automatically. This will avoid to run the application locally and just use the production application

The idea is to have a process where users can review and log all the changes applied to the schema. Unlike most cms this will allow us to keep track of all the modifications and to avoid having users adding and removing fields without any discussion