Release tags in appdata

[nedrichards]

We now have appdata, yay! However we need to update the release tags on each release. I think that means we insert @APP_VERSION@ and the correct date into the system and leave it to do its thing - however there might be some finicityness from appstream-glib that we'll need to keep an eye on (it doesn't like it if it thinks release tags are in the future).