Add admin flag to user model

- app/models/user.js: add admin flag to bypass owner check
- app/models/page.js: fix user relation name
- app/models/post.js: fix user relation name
- app/routes/api/*.js: fix owner check to allow admin users
- bin/shell: add admin flag to administrator user
- bin/shell: update userAdd() to allow admin flag setting

Signed-off-by: Boris HUISGEN <bhuisgen@hbis.fr>