Move image-container CSS to the polls component

This is the only place where we use the `image-container` HTML class.

app/assets/stylesheets/participation.scss

@@ -1390,12 +1390,6 @@
   }
 }
 
-.image-container {
-  background: #fafafa;
-  overflow: hidden;
-  position: relative;
-}
-
 .public .poll {
   border: 1px solid $border;
   margin-bottom: calc($line-height / 2);

app/assets/stylesheets/polls/poll.scss

@@ -4,8 +4,14 @@
     @include breakpoint(medium) {
       padding: 0 calc($line-height / 2) 0 0;
     }
+  }
 
-    .image-container img {
+  .image-container {
+    background: #fafafa;
+    overflow: hidden;
+    position: relative;
+
+    img {
       height: 100%;
       max-width: none;
       position: absolute;