PERF: bail out of expensive post validations

Whenever a post already failed "lightweight" validations, we skip all the expensive validations (that cooks the post or run SQL queries) so that we reply as soon as possible.

Also skip validating polls when there's no "[/poll]" in the raw.

Internal ref - t/115890

plugins/poll/lib/poll.rb

@@ -318,6 +318,9 @@ class DiscoursePoll::Poll
     # creation process. `raw` could be nil here.
     return [] if raw.blank?
 
+    # bail-out early if the post does not contain a poll
+    return [] if !raw.include?("[/poll]")
+
     # TODO: we should fix the callback mess so that the cooked version is available
     # in the validators instead of cooking twice
     raw = raw.sub(%r{\[quote.+/quote\]}m, "")