js: Convert pm_conversations module to ES6.
js: Convert pm_conversations module to ES6.

This was converted automatically using a jscodeshift script followed
by running eslint and prettier to convert let -> const (whenever
applicable) and removing "use strict;".

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