Lazy load school and group images

[linusgke]

• [x] Implement java-script lazy loading for images
• [x] No longer use deprecated onerror attribute
• [x] No longer use placehold.co placeholders

[lchristmann]

Suggestion for addressing point 2 and 3:

• implement the same behaviour, by doing explicit checks in PHP like

<? if (image_exists_in_filesystem()) ?>
    displayThisImage
<? else ?>
    displaySomePlaceholderImageFromOurImgFolder
<? elseif ?>

[lchristmann]

Implementation:

• download all the used placeholders into the assets folder
• replace the onerror attribute with custom php logic

Push & Deployment:

• push the code & deploy it to the server