split common.py into submodules

common.py is too long, it makes contributing hard. I think it should be broken up into smaller submodules.

for starters: mv fdroidserver/common.py fdroidserver/common/__init__.py

Then we can identify groups of features and untangle them into separate files. (e.g. config, network related code for fdroid deploy, reproducible builds related code, etc.)

I also think code like metadata.py would make more sense as a common submodule, because it's used by several subcommands.