replace linkchecker with W3C checklink

[peterstadler]

This PR replaces updates our Guidelines-Link-Check job to use the W3C checklink rather than the old linkchecker. Changes are made to the Docker image as well as to the job configuration to achieve the switch.

While it's working in general (and is already deployed at https://jenkins2.tei-c.de/job/Guidelines-Link-Check/), some things could need some polish (or are debatable):

• the log output from checklink is spit out to the Jenkins console without further transformation. To make this properly fail or validate we need some addition to the log parsing rules or we reemploy some sort of post processing of the logs
• the list of exceptions is rather small now. If I read the man page correctly, the -X parameter is allowed only once, so the exceptions would need to be turned into one ugly (long) regex

[martindholmes]

Thanks for working on this. I use the W3C link checker on various projects, but I don't run it locally; I tend to build pages of links for the W3C service to check, to avoid flooding my network with hundreds of requests.

I tested the install stuff on my Ubuntu box, and the only oddity was that I had to make an explicit choice here:

Package perl-modules is a virtual package provided by:
  perl-modules-5.26 5.26.1-6ubuntu0.3
  perl-modules-5.30 5.30.0-9ubuntu0.2
You should explicitly select one to install.

E: Package 'perl-modules' has no installation candidate

I chose the later one. I'd like to see the checks run on your much faster setup before I reconfigure mine, because I've seen link-check jobs take days in the past (I think some throttling happens on my network), but I'm OK with merging this. Should @raffazizzi have a look too and test on his setup?

[peterstadler]

Since it's not really mission critical we might wait for @raffazizzi to have a look, otherwise it's not mission critical so we could just merge it (it's running smoothly on my second instance).

Regarding the perl-modules: I don't know how that's resolved for the Debian based Docker image in detail but since the image is built ok, I think that's a non-issue (right now).