Add "boards" context processor

This adds the "boards" context data to all pages, so they can render the
boards nav page.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

board/context_processors.py

@@ -1,5 +1,10 @@
 from django.conf import settings as django_settings
+from board.models import Board
 
 
 def settings(request):
     return {"settings": django_settings}
+
+
+def boards(request):
+    return {"boards": Board.objects.filter(readonly=False)}