app/repo: Load repository metadata from ostree-metadata ref if possible

Newer repositories will store metadata there, rather than in the summary
file (although the summary file will still be updated where possible for
backwards compatibility).

Signed-off-by: Philip Withnall <withnall@endlessm.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f