Merge branch 'debug-builds' into 'master'

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.

See merge request !49