MNT: refactoring of JS and HTML into manageable parts

[jsheunis]

This PR:

• Closes ENH: investigate/implement separation between vue templates and index.html #175
• Removes component template code from index.html and puts this into separate template files in templates/, mainly through the use of vue async components and router lazy loading
• Separates the single file javascript code into several files to create structure, all remaining in the global scope and being run in order from index.html
• Updates setup to add templates/ to application data
• Updates relevant tests