Separate out default HTML page from server

Starting to adapt my own OpenTrailView project to use GeoVisio now, with the aim of (hopefully) addressing some of the issues I raised a month or so ago.

One thing which I thought about regarding the server is: should the server's create_app() set up the routes for the default HTML pages (main.html and viewer.html), or should those routes be separated out into a different module?

The reason I suggest this is that it would, to my mind, be more modular to just have the core server deal with the API itself, and not to be coupled to any particular front-end HTML. So a developer can then import the server module (containing API functionality only) and serve their own front-end, as necessary, by adding additional routes to the Flask object returned from create_app().

Then, in the repo, a default module to serve main.html and viewer.html could be created, which makes use of the server.