server: --sync-from-local-copy-dir for updating from offline signing repo

server: --sync-from-local-copy-dir for updating from offline signing repo

To support a fully offline build/signing machine, there is the "local copy
dir".  The repo is generated on the offline machine and then copied to a
local dir where a thumb drive or SD Card is mounted.  Then on the online
machine, using `fdroid server update --sync-from-local-copy-dir` allows
the whole server update process to happen in a single command:

0. read config.py on online machine's repo
1. rsync from the local_copy_dir to the current dir
2. copy to serverwebroot, awsbucket, etc.