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_model.py

@@ -175,6 +175,21 @@ class DBModel:
       self.cursor.fetchall()
     ))
 
+  def payment_session_redirect(self, email, id):
+    self.cursor.execute("SELECT redirected FROM payment_sessions WHERE email = %s AND id = %s", 
+      (email, id)
+    )
+    row = self.cursor.fetchone()
+    if row:
+      self.cursor.execute("UPDATE payment_sessions SET redirected = TRUE WHERE email = %s AND id = %s", 
+        (email, id)
+      )
+      self.connection.commit()
+      return row[0]
+    
+    return None
+
+
   def consume_payment_session(self, payment_type, id, dollars):
     self.cursor.execute("SELECT email, dollars FROM payment_sessions WHERE id = %s AND type = %s", (id, payment_type))
     row = self.cursor.fetchone()