Added commands and variables to run something when client connect/disconnect

[skipperro]

• Added support for SELKIES_START_AFTER_CONNECT and SELKIES_START_AFTER_DISCONNECT variables
• Added support for --start-after-connect and --start-after-disconnect start arguments
• Removed duplicated base64 import in signalling_web.py

Additional context

• I confirm that this pull request is relevant to the scope of this project. If you know that upstream projects are the cause of this problem, please file the pull request there.
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[ehfd]

I will review this but will take weeks before I gain enough dedicated time. Thank you!

[ehfd]

The overall code style by Dan Isla prefers using a Python-native variable over os.environ.