upgrade: Switch to using crudini to update zulip.conf contents.
upgrade: Switch to using crudini to update zulip.conf contents.

Using `config_file.write()` only writes out what python stored of the
file; as such, it strips all comments and whitespace.

Use `crudini --set`, which only modifies the line whose contents are
changed.

zulip/zulip,andersk/zulip,punchagan/zulip,showell/zulip,punchagan/zulip,showell/zulip,zulip/zulip,andersk/zulip,hackerkid/zulip,zulip/zulip,eeshangarg/zulip,rht/zulip,eeshangarg/zulip,kou/zulip,zulip/zulip,kou/zulip,hackerkid/zulip,rht/zulip,hackerkid/zulip,zulip/zulip,eeshangarg/zulip,kou/zulip,showell/zulip,rht/zulip,punchagan/zulip,punchagan/zulip,andersk/zulip,hackerkid/zulip,showell/zulip,hackerkid/zulip,rht/zulip,andersk/zulip,kou/zulip,kou/zulip,showell/zulip,kou/zulip,zulip/zulip,punchagan/zulip,showell/zulip,hackerkid/zulip,andersk/zulip,andersk/zulip,kou/zulip,hackerkid/zulip,eeshangarg/zulip,showell/zulip,eeshangarg/zulip,eeshangarg/zulip,andersk/zulip,zulip/zulip,eeshangarg/zulip,punchagan/zulip,punchagan/zulip,rht/zulip,rht/zulip,rht/zulip