Improve maintainability of this app

[rubensworks]

Right now, several things are hardcoded in the application, which may make things hard to maintain if some Comunica-related things change.

I would suggest to improve this by looking in these directions:

• Create separate template files for mediator and actor configs (also package.json), so we can easily change these should Comunica's config structure change.
• Move URLs to a config file (such as https://linkedsoftwaredependencies.org/)
• Dynamically determine latest package versions (in particular, @comunica/actor-init-sparql is now fixed at 1.17.0, but this changes quite often, also components.js) (Note that the linkedsoftwaredependencies also include this version (only the major part, so ^1.17.0 -> ^1.0.0)
• ...