Get rid of JVM option dataverse.fqdn and validate dataverse.siteUrl on boot

During my time with Dataverse and being on the IRC channel every day, I noticed a lot of confusion, errors and waste of time created by two things:

1. dataverse.fqdn and dataverse.siteUrl settings are confusing when to use which, what are the consequences of their use etc. They get mixed up pretty much.
2. dataverse.siteUrl not containing a proper URL, but only the FQDN part.

I propose with the strong support of @pdurbin and @donsizemore that

1. we should remove dataverse.fqdn and instead concentrate on one, non-confusing setting dataverse.siteUrl.
2. That setting should be validated at boot time, so people fail early when they don't use a proper URL for it.

This is a rather small thing to do. It actually will reduce code complexity, make the docs more precise and easier to understand for newbies. I suggest this is a small issue and volunteer to create a PR.

Back in the days the docs had been extended to better explain the usage, see #4887

The issue names no files or tests. Start by locating definitions and usages of dataverse.fqdn and dataverse.siteUrl, then trace the application boot configuration path. Done means dataverse.fqdn is removed, dataverse.siteUrl is validated as a proper URL at boot, and the affected documentation and tests reflect the single setting.