Addresses requests to make it easier to avoid conflicts between the
Horizon dashboard and http services that might already be running on
the machine.

Configurable via snap config. Exposing via arguments to .init and
testing post init configuration is left for a separate PR.

Eventually, these may move to non standard ports by default. This PR
sets the stage for that, but further discussion is needed before we
decide whether to implement.

(This commit also contains a sneaky fix for the username display at the
end of the launch script.)

Closes-Bug: 1814829
Change-Id: If728d6ec8024bca4d3e809637fbdcc03ed4e6934