Remove Vue.js frontend

[gmmcal]

Is your feature request related to a problem? Please describe.
Vue.js is a very interesting framework, but not having a server side rendering might affect the traceability of the content. Adding server side rendering for the frontend makes no sense, because rails is a fullstack framework.

Describe the solution you'd like

• Remove all Vue components and replace them with plain HTML content.
• Remove all Vue component tests and write meaningful frontend tests (as required).
• Remove markdown feature and use ActiveText instead.

This issue relates to #111.