Add option to disable access to api dashboard

Hi, this MR adds the following:

• an nginx rule to return a 403 on /api/admin
  • default is false to keep behavior
• a question to the install.sh script
  • it's only asked in the customization dialog
  • default is false

defaults/main.yml

@@ -6,6 +6,7 @@ funkwhale_static_path: /srv/funkwhale/data/static
 funkwhale_music_path: /srv/funkwhale/data/music
 funkwhale_config_path: /srv/funkwhale/config
 funkwhale_external_storage_enabled: false
+funkwhale_disable_django_admin: false
 funkwhale_username: funkwhale
 funkwhale_database_managed: true
 funkwhale_frontend_managed: true