Merge branch 'request/monospace_gfm_input' into 'master'

Use monospace fonts for GFM inputs

This is an MR for [this feedback suggestion](http://feedback.gitlab.com/forums/176466-general/suggestions/5593468-use-monospace-font-in-wiki-edit-content-textarea).

I'm not sure exactly where the best place for the fix is, but I opted for the typography.scss file.  Basically, all ``textarea.js-gfm-input`` elements are styled with ``font-family: $monospace_font;``.

*PS: this being my first MR to gitlab, I apologize if I missed any steps in the process; let me know what's missing and I'll correct it here & in future MRs*

app/assets/stylesheets/generic/typography.scss

@@ -120,3 +120,11 @@ a:focus {
 .md {
   @include md-typography;
 }
+
+/**
+ * Textareas intended for GFM
+ *
+ */
+textarea.js-gfm-input {
+  font-family: $monospace_font;
+}