Make multi tenancy optional

[flyaruu]

It was always meant to be optional, but we use mostly use multi tenant instances, and over time some features broke for single tenant.

As far as I know:

• The tester demands a tenant
• Testing entities from the entitydocumentation only works when supplying a tenant. I also think that the authentication screen in the entitydocumentation should use a dropdown of tenants instead of a text field.
• Calling an entity does not work without a tenant
• I'm unsure of all authentication options, I suspect OAuth also does not work without a tenant

Until now we've solved it by just creating a 'dummy' tenant, which works but it adds some noise to the user experience