Add stock images, closes #2482

resources/vue/components/stock-images/Thumbnail.vue

resources/vue/components/stock-images/ThumbnailCard.vue

resources/vue/components/stock-images/UploadBox.vue

resources/vue/components/stock-images/UploadDialog.vue

resources/vue/components/stock-images/colors.js

resources/vue/components/stock-images/components.js

resources/vue/components/stock-images/filters.js

resources/vue/components/stock-images/format.js

resources/vue/courseware-index-app.js

resources/vue/courseware-shelf-app.js

resources/vue/plugins/stock-images.js

resources/vue/store/courseware/courseware-shelf.module.js

resources/vue/store/courseware/courseware.module.js

resources/vue/store/stock-images.js