Multi-Tenancy support

[ccoenen]

Some of us run more than one instance, so it would be good to support that, too.

Ideally, we'd simply query all of their histories (this only takes a fraction of a second, so it is preferable to adding yet another click to the workflow) and order them by timestamp.

A CodiMD instance should be identified by

• a human readable name (defaults to domain name)
• a color