Merge branch 'fix-mixed-color-preview' into 'master'

Fix bug where notes were being rendered with wrong color preferences due to caching

### What does this MR do?

This MR adds the user's color preferences as part of the caching key for notes.

### Why was this MR needed?

This should fix a bug introduced via 4d80360b where notes were being loaded from the cache without regard to the current user's color scheme.

### What are the relevant issue numbers?

Closes #2008

See merge request !994

app/views/projects/notes/_note.html.haml

@@ -56,7 +56,7 @@
 
 
       .note-body{class: note_editable?(note) ? 'js-task-list-container' : ''}
-        = cache [note, 'markdown'] do
+        = cache [note, 'markdown', user_color_scheme_class] do
           .note-text
             = preserve do
               = markdown(note.note, {no_header_anchors: true})