refactor: move release changelog workflow to releases repo

The workflow triggers on `release: published` which only fires from the
default branch. Since the releases repo default branch is `main` (not
`master` synced from server), the workflow never triggered from here.

Move it to nextcloud-releases/server where it belongs alongside other
release infrastructure. Also adds HTML changelog generation and attaches
it as a release asset for the marketing team.

Signed-off-by: John Molakvoæ (skjnldsv) <skjnldsv@protonmail.com>

Author: skjnldsv