Improve developer experience with html templates

[Ranthalion]

As a developer, I want to use a template engine for the web interface.

The current implementation seems difficult to extend and improve because the html is generated inline in a single file. A template engine, for example jinja, separates the html template from server logic, making it easier to extend and maintain. Flask has template support built in, as well as a routing system.

@patvdleer has a branch at flask_presets that shows how the web engine could be implemented.

@florianfesti I'd be happy to help with a dedicated PR to introduce flask and jinja if there is any appetite to go that direction.