support plain paths as a serverwebroot

For use cases where there is a web server running on the same machine where the fdroid update is being run, allow plain paths in the serverwebroot list. This is useful for debug repos from build servers, like:

https://dev.guardianproject.info/fdroid

This also includes small change in the fdroid server update delete behavior: it delays the delete phase until after the new APKs are uploaded. This is keep things working for a long as possible while the repo is being updated.