Flatpak: use YAML rather than JSON in the manifest.

This allows us to make the manifest more concise and readable.
1 job for feature/yamlize-flatpak in 16 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build Flatpak
failed #37432


Name Stage Failure
flatpak:master Build Flatpak
Cloning repository...
Cloning into '/builds/GNOME/gnome-contacts'...
Checking out ed66836b as feature/yamlize-flatpak...
Skipping Git submodules setup
Checking cache for default-1...
Successfully extracted cache
$ flatpak-builder --stop-at=gnome-contacts app data/flatpak/org.gnome.Contacts.yml
Can't parse 'data/flatpak/org.gnome.Contacts.yml': <data>:1:6: Parse error: unexpected identifier `app-id', expected value
ERROR: Job failed: exit code 1