Add automatic broken link checking

[sSwerk]

Add support for automatic broken link checking, see here.

[sSwerk]

First manually triggered run indicates the following issues:

Run bundle exec htmlproofer --empty_alt_ignore --assume-extension  ./_site
NOTE: nokogumbo: Using Nokogiri::HTML5 provided by Nokogiri. See https://github.com/sparklemotion/nokogiri/issues/2205 for more information.
Running ["ScriptCheck", "LinkCheck", "ImageCheck"] on ["./_site"] on *.html... 


Checking 620 external links...
- ./_site/acknowledgements/index.html
Ran on 155 files!
  *  External link http://david.abcc.ncifcrf.gov/ failed: response code 0 means something's wrong.

             It's possible libcurl couldn't connect to the server or perhaps the request timed out.

             Sometimes, making too many requests at once also breaks things.
             Either way, the return message (if any) from the server is: Couldn't resolve host name
- ./_site/apps/embeding-structure-explorer/index.html
  *  External link https://fonts.gstatic.com failed: 404 No error
- ./_site/former-caleydo/event/2014/07/26/vis-papers/index.html
  *  External link https://ieeevis.org/ failed: got a time out (response code 0)
- ./_site/former-caleydo/event/2015/04/22/open_phd_position_JKU/index.html
  *  External link http://data.jku-vds-lab.at/jobs/2015-04_JKU_ResearchAssistant.pdf failed: 404 No error
- ./_site/former-caleydo/event/2015/06/19/open_positions_utah/index.html
  *  External link http://www.sci.utah.edu/75-sci-opportunities/post-doc/536-post-doc-lex.html failed: 404 No error
- ./_site/former-caleydo/event/2016/03/01/open_positions_harvard/index.html
  *  External link https://dbmi.hms.harvard.edu/careers/software-engineers/visualization-software-developer failed: 404 No error
- ./_site/persons/eckelt/index.html
  *  External link https://www.linkedin.com/in/klaus-eckelt failed: 999 No error
- ./_site/persons/kawan/index.html
  *  image has no src or srcset attribute (line 92)
- ./_site/persons/streit/index.html
  *  External link https://www.linkedin.com/in/marcstreit failed: 999 No error
- ./_site/persons/walchshofer/index.html
  *  External link https://www.linkedin.com/in/conny-walchshofer failed: 999 No error
- ./_site/publications/2013_infovis_lineup/index.html
  *  External link https://www.iu.qs.com/university-rankings/world-university-rankings/ failed: response code 0 means something's wrong.
             It's possible libcurl couldn't connect to the server or perhaps the request timed out.
             Sometimes, making too many requests at once also breaks things.
             Either way, the return message (if any) from the server is: SSL peer certificate or SSH remote key was not OK
- ./_site/publications/2014_bmc_furby/index.html
  *  External link https://www.broadinstitute.org/cgi-bin/cancer/datasets.cgi failed: 502 No error
- ./_site/publications/2017_bmc_clustering/index.html
  *  External link http://data.jku-vds-lab.at/papers/2017_bmc_clustering.pdf failed: 404 No error
- ./_site/publications/2019_eurova_rubik/index.html
  *  External link https://github.com/JKU-ICG/rubiks-cube-demonstrator failed: 404 No error
- ./_site/publications/2020_imimic_projective-latent-space-deculttering/index.html
  *  External link https://dx.doi.org/10.1007/978-3-030-61166-8_2 failed: 302 Number of redirects hit maximum amount
- ./_site/publications/2021_forecasting_model_sinter/index.html
  *  External link https://www.scopus.com/record/display.uri?eid=2-s2.0-85092267170&origin=inward&txGid=5f94bfde7a78639aa60327d0e70e1a23 failed: 302 Number of redirects hit maximum amount
- ./_site/publications/2021_preprint_dashboard_onboarding/index.html
  *  External link http://data.jku-vds-lab.at/papers/2021_preprint_dashboard-onboarding.pdf failed: 404 No error
  *  External link http://data.jku-vds-lab.at/papers/https://osf.io/zsb5a/ failed: 404 No error
- ./_site/publications/2021_tvcg_fuzzy-spreadsheets/index.html
  *  External link http://data.jku-vds-lab.at/papers/https://osf.io/8ypux/ failed: 404 No error
- ./_site/releases/0_domino_3_1_5/index.html
  *  External link https://support.amd.com/us/gpudownload/Pages/index.aspx failed: 403 No error
- ./_site/team/index.html
  *  External link https://www.jku.at/en/institute-of-computer-graphics/about-us/team/moritz-schoefl/ failed: 404 No error
- ./_site/templates/person-template/index.html
  *  internal image /assets/images/team_colored/doe.jpg does not exist (line 92)

HTML-Proofer found 22 failures!
Error: Process completed with exit code 1.