implement content-security-policy, static assets cache bust, and fix

stripe back button ratchet issue

because the only way to use stripe checkout is to run their proprietary
JS, and we arent using a SPA, naturally what happens is, when you land
on the stripe payment page if you hit the back button it goes back to
the same page where you got re-directed to stripe. this commit fixes
that.

capsulflask/db.py

@@ -40,7 +40,7 @@ def init_app(app):
   hasSchemaVersionTable = False
   actionWasTaken = False
   schemaVersion = 0
-  desiredSchemaVersion = 4
+  desiredSchemaVersion = 5
 
   cursor = connection.cursor()