Cleaned up the changelog

I've added an `h1` title and I've updated all the releases that were using `h1` to `h2`.

Hopefully, this would enable the "changelog" tab in the application details like we can see on other apps such as Contacts, Mail, News, etc...
I've based my changes on Contacts' changelog: https://github.com/nextcloud/contacts/blob/master/CHANGELOG.md



Signed-off-by: Romain Failliot <romain.failliot@foolstep.com>