Look for custom cert/key, fall back to self-signed

TODO

• look for custom .key .cert on server start
• rename plom.key to plom-selfsigned.key to match self-signed cert.
• remove the per-call verify=False in favour of a per-session selection
• Default to true
  • but pop-up a dialog to allow users to ignore cert errors
  • [ ]
• Env var overrides:
  • document how to use a sneaker-netted cert: roughly,

    export REQUESTS_CA_BUNDLE=/path/to/servers_cert_file.crt
    python3 -m plom.client
    python3 -m plom.scan
    # etc

  • this also works for the command-line tools
• Avoiding aggravating our developers: if "dev" in plom.__version__ then fallback to no-SSL-verification sans popup dialog (just log a warning).
• Do we need to add -k to all command-line tools? This matches curl -k https://... but might require some deeper refactoring (because the command line tools build msgr objects all over the place).
• Quick fix: add a PLOM_SSL_NO_VERIFY env var. (I have haven't found an env var-based technique to disable SSL verification at requests or urllib3 level.)