Use What's new service instead of Pastebin

[defagos]

We now provide release notes information via MMF, and should therefore stop retrieving them from Pastebin.

[defagos]

As discussed with @pyby, we should probably move to a single file listing all versions, even those more recent than the one installed (clearly identified in the list). This way users can discover what they are missing by not upgrading to the most recent version.

Moreover, we should only display an in-app notification when a new version is launched for the first time. If the user taps on the notification the list should be displayed, otherwise not. This is less intrusive and avoids displaying too many screens at startup (most notably when an onboarding is displayed as well).

This has been briefly discussed with Michael and added to our backlog.

[pyby]

Release notes has been moved to Github pages. See #236.