add Blubber block for Courseware, fixes #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

...es/assets/stylesheets/scss/courseware/blocks/blubber.scss

resources/assets/stylesheets/scss/courseware/variables.scss

...e/components/courseware/blocks/CoursewareBlubberBlock.vue

...components/courseware/blocks/CoursewareBlubberComment.vue

.../components/courseware/blocks/CoursewareBlubberThread.vue

.../components/courseware/containers/container-components.js

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

tests/jsonapi/BlubberThreadsCreateTest.php