backend: provide helper script copr-assure-permissions
I'd like to use this script to traverse the whole backend result
directory, and fix the ownership to 'copr:copr' and permissions to
0644 on regular files, and 0755 for directories.
We can run this script from time to time on the result directory, and
the stderr should stay clean in the future, unless something suspicious
is happening.
Relates: #1804