add CoursewareBlubberBlock, re studip/studip#726

lib/classes/JsonApi/RouteMap.php

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

lib/classes/JsonApi/Routes/Blubber/ThreadsCreate.php

lib/classes/JsonApi/Routes/Blubber/ThreadsUpdate.php

lib/models/Courseware/BlockTypes/BlockType.php

lib/models/Courseware/BlockTypes/Blubber.json

lib/models/Courseware/BlockTypes/Blubber.php

resources/assets/stylesheets/scss/courseware.scss

...rces/vue/components/courseware/CoursewareBlubberBlock.vue

...es/vue/components/courseware/CoursewareBlubberComment.vue

...ces/vue/components/courseware/CoursewareBlubberThread.vue

resources/vue/components/courseware/container-components.js

resources/vue/courseware-index-app.js

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