Make the frontend notice when the backend was updated (so it can reload itself)

[LukasKalbertodt]

As Tobira is a SPA, it can easily happen that a user keeps a tab open for a long time, never reloading index.html and thus the JS files. This means, the frontend and backend can version mismatch, leading to strange API errors. It would be more elegant if the frontend would notice such an update and then either prompt the user to reload or automatically reload. I don't expect the frontend to regularly ping, but it could query the backend version in every API request that it performs anyway.