Do not allow selection of line numbers in code blocks (#594)

Author: jessetan

sass/_theme_rst.sass

@@ -70,6 +70,8 @@
     margin: 0
     padding: $base-line-height / 2 $base-line-height / 2
     font-family: $code-font-family
+    user-select: none
+    pointer-events: none
   div[class^='highlight'] pre
     white-space: pre
     margin: 0