Add stock images, closes #2482

lib/classes/JsonApi/Schemas/StockImage.php

lib/classes/StockImages/PaletteCreator.php

lib/classes/StockImages/Scaler.php

lib/models/Courseware/BlockTypes/Headline.json

lib/models/Courseware/StructuralElement.php

lib/models/StockImage.php

lib/navigation/ContentsNavigation.php

package-lock.json

package.json

public/assets/images/checkered-background.png

public/pictures/stock-images/.gitkeep

resources/assets/javascripts/bootstrap/stock-images.js

resources/assets/javascripts/entry-base.js

resources/assets/javascripts/lib/stock-images.js

resources/vue/components/ActiveFilter.vue

resources/vue/components/SearchWithFilter.vue

...urces/vue/components/courseware/CoursewareFileChooser.vue

...ces/vue/components/courseware/CoursewareHeadlineBlock.vue

...ces/vue/components/courseware/CoursewareSearchResults.vue

...rces/vue/components/courseware/CoursewareSearchWidget.vue