Add External Data Checker Functionality

[RayJW]

This adds External Data Checker functionality to help with creating new releases. This should make the process a lot easier, since new releases will automatically trigger a PR, which can simply be merged.

[flathubbot]

Started test build 163790

[flathubbot]

Build 163790 failed

[RayJW]

Okay, I see the problem. So this probably not possible to fully automate since the generated-sources.json can't be pulled from upstream like this because of the order the steps are executed. This could be resolved by adding a workflow to the repo that updates this file from upstream when a new version is released, like others have done. Although, the workflow could be simpler as it only has to pull the upstream file. The other option would be to just leave it like this and manually update the file for every PR. The workload would still be reduced as the bot does everything except that step, which means the build would always fail until a maintainer does that by hand for every PR from Flathub Bot (reference).

The other option would be building the package upstream and then simply packaging the binary here, as that would be way less of a hassle. But I understand that this would be the less desirable solution.

[flathubbot]

Started test build 163822

[flathubbot]

Build 163822 successful
To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/146906/com.notesnook.Notesnook.flatpakref

[flathubbot]

Started test build 165761

[flathubbot]

Build 165761 failed

[RayJW]

This won't build properly until the Flatpak uses a tagged commit again. I guess merging can wait until a new release is out, if the devs are interested in this PR at all.