widgets: Add support for clickable links.

The change was made to support links in polls, as mentioned in
issue zulip#12947. We used markdown renderer to render
the link content, and parsed out any unnecessary p tags.
We changed javascript and hbs files so that they properly
render the content. Tested locally whether the links work,
in addition to checking for XSS vulnerbilities.
Everything tested worked, and no vulnerabilities
discovered. Double check that there are no XSS
issues.

Fixes: #1294o7

zerver/lib/actions.py

@@ -1819,7 +1819,6 @@ def do_add_submessage(realm: Realm,
         content=content,
     )
     submessage.save()
-    print("here")
     event = dict(
         type="submessage",
         msg_type=msg_type,