FIX: Validate poll arguments. (#6740)

* FIX: Validate number poll.
* FEATURE: Poll's min can be 0.
* FIX: Fix URL to user profile.

plugins/poll/test/javascripts/acceptance/polls-test.js.es6

@@ -2023,6 +2023,8 @@ test("Public number poll", async assert => {
     "it should display the right number of voters"
   );
 
+  assert.ok(find(".poll-voters:first li:first a").attr("href"), "user URL exists");
+
   await click(".poll-voters-toggle-expand:first a");
 
   assert.equal(