Detect if Multisite is not enabled.

[ideadude]

We do already check if the plugin is being network activated, but we should also check if multisite is enabled when this is activated on a single site.

The checkout still adds fields for site/etc, but won't work.

[ideadude]

Alternatively, we can/should redirect to detailed documentation for the add on that suggests getting a developer. We don't intend for novices to be able to set this up yet.