Provide hasPreview in files_versions DAV API

[artonge]

This allows to no request non-existing previews I also set some properties to the <img> element to reduce preview loading to what the browser think is necessary

[artonge]

/backport to stable27

[backportbot-nextcloud[bot]]

The backport to stable27 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable27
git pull origin stable27

# Create the new backport branch
git checkout -b fix/foo-stable27

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts. Resolve them.
git cherry-pick abc123

# Push the cherry pick commit to the remote repository and open a pull request
git push origin fix/foo-stable27

More info at https://docs.nextcloud.com/server/latest/developer_manual/getting_started/development_process.html#manual-backport