Wagtail packaging fixes (#8030)

MANIFEST.in

@@ -1,8 +1,10 @@
 include LICENSE *.rst *.txt *.md
 graft wagtail
-prune wagtail/wagtailadmin/static_src
-global-exclude __pycache__
+prune **/__pycache__
+prune **/static_src
 global-exclude *.py[co]
 global-exclude *.stories.js
 global-exclude *.stories.tsx
 recursive-exclude wagtail/ *.md
+global-exclude .gitignore
+global-exclude wagtail-icomoon.json