Add KaTeX support

src/assets/scss/_base.scss

@@ -1,5 +1,6 @@
 @charset "UTF-8";
 
 @import "normalize-scss";
+@import "~katex/dist/katex.min.css";
 @import "~prismjs/themes/prism-solarizedlight.css";
 @import "base/generic";

src/components/Post/Content/Content.module.scss

@@ -44,13 +44,16 @@
       text-decoration: underline
     }
 
-    & *:not(div) {
-      width: 100%;
+    & * {
       max-width: $layout-post-width;
       margin-left: auto;
       margin-right: auto
     }
 
+    & img {
+      max-width: 100%;
+    }
+
   }
 
 }