js: Purge pm_conversations module from window.

All the changes are done using a script followed by updates to Node
tests and running eslint and prettier to fix formatting issues.

static/js/ui_init.js

@@ -8,6 +8,7 @@ const render_edit_content_button = require("../templates/edit_content_button.hbs
 
 const emojisets = require("./emojisets");
 const markdown_config = require("./markdown_config");
+const pm_conversations = require("./pm_conversations");
 
 // This is where most of our initialization takes place.
 // TODO: Organize it a lot better.  In particular, move bigger