Temporary disable sign in link on header

So far we don't have any functionality for sign in and it might cause
some kind of misunderstanding. To avoid this this commit dissables
sing-in link.

Author: lotrien

src/styles/Header.styl

@@ -40,6 +40,7 @@
     &-x
       color: text-green
   &-sign-in
+    pointer-events: none
     margin-right: 20px
     & a
       display: flex