Fix inner borders in polls administration tables

The rows in these tables were using the styles from the `.poll`
selector, and the `position: relative` property defined there caused the
inner borders to disappear in some browsers (like Firefox).

So we're adding the `public` class to the selector; this way, it doesn't
affect elements in the admin section.

Even though it's only necessary to add the `.public` prefix to the
`.poll` selector in one place in order to fix this issue, we're doing it
everywhere for consistency.

app/assets/stylesheets/participation.scss

@@ -1515,7 +1515,7 @@
   position: relative;
 }
 
-.poll,
+.public .poll,
 .poll-question {
   border: 1px solid $border;
   margin-bottom: calc(#{$line-height} / 2);

app/assets/stylesheets/polls/poll.scss

@@ -1,4 +1,4 @@
-.poll {
+.public .poll {
   &.with-image {
 
     @include breakpoint(medium) {