Accept any character in self-hosted .env values

Self-hosted operators can now put any password or secret in their configuration file, including the characters that used to abort an update.

• Fixed the deploy script failing on a .env value containing a backtick, a dollar sign, a space or a quote
• Kept the quotes an operator adds around a value when the script rewrites .env on each deployment
• Added a clear error with the line number when a value is badly quoted, instead of a silently shortened value
• Documented how to quote special characters on the self-hosting configuration page