Improve md typography

Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/assets/stylesheets/generic/typography.scss

@@ -88,9 +88,6 @@ a:focus {
 .wiki {
   @include md-typography;
 
-  font-size: 14px;
-  line-height: 1.6;
-
   /* Link to current header. */
   h1, h2, h3, h4, h5, h6 {
     position: relative;

app/assets/stylesheets/main/mixins.scss

@@ -84,6 +84,9 @@
 }
 
 @mixin md-typography {
+  font-size: 14px;
+  line-height: 1.6;
+
   img {
     max-width: 100%;
   }