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

[poikilotherm]

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.

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

[poikilotherm]

Incredible @donsizemore just pinged me on IRC that we need to take care of a few code places, dust off things and streamline to use the proper internal APIs.

https://github.com/IQSS/dataverse/search?q=fqdn&unscoped_q=fqdn

[Asbjoedt]

I think this would be a good thing to implement, since it will reduce possible confusion/decision-making about which option to configure, if necessary to change a value.

[cmbz]

To focus on the most important features and bugs, we are closing issues created before 2020 (version 5.0) that are not new feature requests with the label 'Type: Feature'.

If you created this issue and you feel the team should revisit this decision, please reopen the issue and leave a comment.