problems: Allow marking a package as renamed
overview: Add a entry for the renamed packages

Now that these aren't problems any more, they are displayed along
regular up-to-date / out-of-date packages.

Nevertheless, it can be interesting to view them all in the same place.
status: Report status of renamed packages
renamed: Allow changing the upstream name of a renamed package

This also allows unmarking a package as renamed, by simply setting the
new name to an empty string.
downstream: Factorize most code

This allows reusing both the same frontend and backend code to mark a
package as downstream-only, or reverting that.