add responsive_images_for_jekyll plugin

[wp99cp]

The file sizes of all our images are huge. For example, the image for the walk-time generator is of size 2030×708px and comes at 1.4 MB, especially on mobile devices such high-resolution images are not needed. We should therefore resize all images and provide different resolutions such that the browser can load the correct one depending on its render size.

As it is a pain to resize images by hand, I would suggest an automated pipeline doing the job for us. As I'm currently evaluating using Jekyll for our Abteilingswebseite as well, I've implemented such a pipeline, see GitRepo. It's still work in progress... but can already be used as a gem module. Feel free to submit issues directly to the repo.

TODO's

• Fix slow initial build time
• Update Readme
• Test automatic build scripts. They will probably fail since you now need to install imagemagick:

/usr/gem/gems/mini_magick-4.11.0/lib/mini_magick/configuration.rb:139:in `cli': You must have ImageMagick or GraphicsMagick installed (MiniMagick::Error)

[maede97]

Looks pretty cool and seems to work! The loading times for the webpage are way down, which is great!

• I think that optipng is the root cause of the slow initial build time. Maybe this can be exchanged?
• For the CI runners: we have to make sure to cache the imgs folder.

[maede97]

I tried adding the libmagick stuff to the pipeline but failed.
Possibly related: minimagick/minimagick#509

[github-actions]

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[wp99cp]

I tried adding the libmagick stuff to the pipeline but failed. Possibly related: minimagick/minimagick#509

I replaced the currently used deploy script with the one from https://github.com/helaili/jekyll-action, as this script allows for convenient pre-build setup commands. This must also be done for the productive build, I've not yet touched this build script. We also need to enable caching for the images and installation directories.