Fix sort order on single notes

The notes are supposed to be presented in the order they appear on a
keyboard, but the "scientific pitch notation" has octaves that start
with C and end with B, so we were accidentally sorting A and B for each
octave before all the other notes in the octave.

_includes/note_variables.html

@@ -64,7 +64,7 @@
   {% assign note_class = "note-" | append: note_base | replace: "#", "-sharp" | replace: "b", "-flat" %}
   {% assign absolute_class = "note-" | append: note_base | append: number | replace: "#", "-sharp" | replace: "b", "-flat" %}
 
-  {% capture sort_key %}{{ number }}{{ letter }}{% if accidental == "#" %}1{% elsif accidental == "b" %}3{% else %}2{% endif %}{% endcapture %}
+  {% capture sort_key %}{{ number }}{% case letter %}{% when 'A' %}H{% when 'B' %}I{% else %}{{ letter }}{% endcase %}{% if accidental == "#" %}1{% elsif accidental == "b" %}3{% else %}2{% endif %}{% endcapture %}
 
   {% assign note_info = "" | split: "|" %}
   {% assign note_info = note_info | push : sort_key %}