Resolves "courseware v-select unable to fully display the list"

This MR fixes #274 (closed).

How it works

• A new courseware component courseware-vue-select which wraps around the v-select is introduced to give more control over it.
• the new component is globally defined, in order to be used everywhere specially in plugins.
• @popperjs/core npm package is required for this PR.
• the new component by default works exactly like v-select.
• now with an ease of adding only append-to-body attribute to the new component, the drop-down event changes to drop-up thanks to popperjs. IMPORTANT: the initial issue was found in Courseware Opencast-Block which also has been taken care of in this MR https://github.com/elan-ev/studip-opencast-plugin/pull/547 and must be merged after this one is merged!