Figure out how to encode `LDAPSearch` objects via environment variables

[timabbott]

While we documented the MANUAL_CONFIGURATION workaround in #344, I think it might be possible to support LDAPSearch objects some other way:

• It might be possible to figure out a string encoding of the Python code that be usefully processed, just like we do arrays/None, with custom code in entrypoint.sh.
• We could just define a few Zulip-specific variable names for the actual arguments to the couple Zulip settings that accept an LDAPSearch object, to at least cover the most common approaches.