Refactoring of the Vue.js application

The goal of that merge request is to convert the Editor Vue.js application into a modern single-file component application.

We'll also introduce two new dependencies:

• axios to build a more generic API client
• vuex to build a generic reactive data storage

Rough steps:

• Convert all pure js components to vue.js single file components (script + template + style)
• Embed all HTML code into these components
• Setup a unique & minimal <Editor> entrypoint in the Django application
• Replace fetch API calls by axios
• Migrate store/part.js actions by a global VueX store
• Replace internal event system by VueX actions
• Replace all this.$parent references by VueX state management
• Replace this.$el calls by local template updates through reactivity