Preview: set font-size on preview content

- Backport of #2349
- When previewing the content in a review, no font size was set. This
resulted in the previewed content being bigger than other text and
therefor creating an noticable inconsistency.
- Set the font size of the previewed content, 14px, this is consistent
with how the content would be rendered.
- `comment-code-cloud` is the class used for the review boxes.
`.ui.tab.markup` means it only applies to the preview tab.

(cherry picked from commit b1aabbf174)

web_src/css/review.css

@@ -153,6 +153,10 @@
   min-height: 168px;
 }
 
+.comment-code-cloud .ui.tab.markup {
+  font-size: 14px;
+}
+
 .comment-code-cloud .ui.tabular.menu {
   margin: 0.5em;
 }