Add stock images, closes #2482

app/controllers/stock_images.php

app/views/stock_images/index.php

cli/studip

composer.json

composer.lock

db/migrations/5.4.7_create_stock_images_table.php

...igrations/5.4.8_add_image_type_to_structural_elements.php

lib/classes/JsonApi/RouteMap.php

...onApi/Routes/Courseware/StructuralElementsImageDelete.php

...onApi/Routes/Courseware/StructuralElementsImageUpload.php

...es/JsonApi/Routes/Courseware/StructuralElementsUpdate.php

lib/classes/JsonApi/Routes/StockImages/Authority.php

lib/classes/JsonApi/Routes/StockImages/StockImagesCreate.php

lib/classes/JsonApi/Routes/StockImages/StockImagesDelete.php

lib/classes/JsonApi/Routes/StockImages/StockImagesIndex.php

lib/classes/JsonApi/Routes/StockImages/StockImagesShow.php

lib/classes/JsonApi/Routes/StockImages/StockImagesUpdate.php

lib/classes/JsonApi/Routes/StockImages/StockImagesUpload.php

lib/classes/JsonApi/Routes/StockImages/ValidationHelpers.php

lib/classes/JsonApi/SchemaMap.php