Improvements to MANIFEST.IN

Reduces size of wheel files to 5.7MB from 9.3MB

MANIFEST.in

@@ -1,4 +1,5 @@
 include LICENSE *.rst *.txt
-recursive-include wagtail *
-recursive-exclude * __pycache__
-recursive-exclude * *.py[co]
+graft wagtail
+prune wagtail/wagtailadmin/static_src
+global-exclude __pycache__
+global-exclude *.py[co]