Links to https://dropbox.com/sh/example/thing in stream descriptions fail to save

The new call to is_static_or_current_realm_url assumes that self.md.zulip_realm is defined: https://github.com/zulip/zulip/blob/ec99c1b58dc319fa3eaeaa883f6e18ed1add425c/zerver/lib/markdown/__init__.py#L584

This is not set for markdown descriptions of streams. As such, putting a URL of https://www.dropbox.com/sh/something/here into a stream description causes the description to fail to save, as rendering the markdown throws a 'NoneType' object has no attribute 'host' error: https://github.com/zulip/zulip/blob/ec99c1b58dc319fa3eaeaa883f6e18ed1add425c/zerver/lib/subdomains.py#L73

We should:

1. Error-proof is_static_or_current_realm_url by fixing the type of realm to be Optional[Realm] and add realm is not None and before the check on line 73
2. Fix do_change_stream_description to pass down the realm to the markdown renderer, so that we do get a realm in that spot, and can make use of it.

[PIG208]

OK, looking into this.

[PIG208]

Note that part of this (typing and None-checking) has been fixed in #23159. #23386 is mainly about fixing do_change_stream_description