vengor: extract common ensure_goproxy code
readme: add more valuable information
slightly improve UI for proxy errors
provide better attachment filenames
html: add modcache button
add updated systemd units
use huey, implement /modcache
vengor: make modes consistent across machines

By utilizing a simplified view of the filesystem inspired by Git, we can
eliminate a few more bytes from potential host-to-host differences.
systemd: set extra env variables
vengor: create cache dir on startup
utils: fix edge case in atomic_write
base.html: use url_for for css links
setuptools: enable include_package_data
setuptools: add MANIFEST.in
external: fix find_packages() detection
add systemd reference files
add reference vengor.service
correct and sort imports