Add support for multiple backends

[innovate-invent]

After #28 a component is needed to take advantage of vuex-orm/vuex-orm-next#53
A list of backends should be stored stored and queried from DNS as URI records. A TXT record for the backend hostname can be queried to retrieve a description.

This list should be provided as a drop down with text entry. Users should be able to manually enter a url, including "localhost".
This component should implement the Vue standardised 'model' attribute.