Have Eloquent scopes and observers skip certain functionality when not within multitenanted context

[ollieread]

The scopes and observers added by BelongsToTenant and BelongsToManyTenants will throw exceptions at various points during the usage of the models that use them. These exceptions, currently, will be thrown while outside multitenanted context, which means that the models can't be used in any sort of admin area.

Both scopes should exit early from within Scope::apply() if the current request isn't marked as being within multitenanted context. Both observers should also exit early for all operations when outside multitenanted context.