Reduced the z-index of the topic-info

When applying the `sticky-top` extension, it inherited a `z-index` of 1020. This meant the topic information box shows up over the version history dropdown box. Oops.

Oddly, however, simply updating the version history to use a higher `z-index` doesn't solve the problem. In fact, the only way I could resolve it was by _lowering_ the z-index of the `topic-info` class.

It's not entirely clear why `sticky-top` comes with such a high `z-index`, nor why it can't be superceded by a higher `z-index`. But, ultimately, it doesn't really matter as we don't need or want the `z-index` on the topic info box to be high, so moving this down to a baseline value of `10` is just fine for now.

Author: JeremyCaney

OnTopic.Editor.AspNetCore/Shared/Styles/Base/_callouts.scss

@@ -24,6 +24,7 @@
   &.topic-info {
     @extend                     .sticky-top;
     top                         : 136px+15px;
+    z-index                     : 10;
     border                      : 1px solid $grey-lighter;
     padding                     : 15px;
     width                       : 100%;