TIC 3265 - option to add links to coursewares in link block and table of contents

...s/JsonApi/Routes/Courseware/CoursewareInstancesUpdate.php

lib/classes/JsonApi/Schemas/Courseware/Instance.php

lib/models/Courseware/BlockTypes/Link.json

lib/models/Courseware/BlockTypes/Link.php

lib/models/Courseware/Instance.php

resources/assets/stylesheets/scss/courseware.scss

...urces/assets/stylesheets/scss/courseware/blocks/link.scss

...es/assets/stylesheets/scss/courseware/layouts/ribbon.scss

...ssets/stylesheets/scss/courseware/layouts/tree-units.scss

.../vue/components/courseware/blocks/CoursewareLinkBlock.vue

...courseware/structural-element/CoursewareToolsContents.vue

...ents/courseware/structural-element/CoursewareTreeUnit.vue

...nts/courseware/structural-element/CoursewareTreeUnits.vue

resources/vue/courseware-index-app.js

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