Visibilité des groupes et URL de redirection externe

• Give visibility to groups created by the syndicate managed of the current contact #86
• adds "external URL" custom field to syndicates #85
• Add "member of" relationship on contribution #84

Fixes and dev stuff:

• Fix permissions on help pages #87
• Refactor user_is_manager_of() function
• Adds usefull functions: cartable_get_contacts_of() & cartable_get_syndicates_of()