filters: Make a backup copy of the filter when it is set and use when needed

We still look at the regular file to pick up changes, but to avoid
issues if the file disappears (for example if it was in a package
that was uninstalled) then we still have something to show so we don't
start accidentally showing unfiltered stuff.

Closes: #2890
Approved by: alexlarsson
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f