UX: make poll content selectable (#13529)

When initially released, the polls had a different design that didn't interact
well with the quote button - https://meta.discourse.org/t/31586

Now that the design has evolved, not being able to select text from inside a poll is
counter productive, so it's enabled again.

plugins/poll/assets/stylesheets/common/poll.scss

@@ -2,8 +2,6 @@ div.poll {
   margin: 1em 0;
   border: 1px solid var(--primary-low);
 
-  @include unselectable;
-
   ul,
   ol {
     margin: 0;