Disabling flask-talisman by default (#7535)

flask-talisman was enabled recently and while it may be virtuous in some
cases, it seems to break things out of the box.

Locally and in dev mode, upon my first redirect it sends to HTTPS and
things it crashes.

I think it should be opt-in, maybe we can recommend turning this on in
production in the docs?

superset/config.py

@@ -613,7 +613,7 @@ class CeleryConfig(object):
 }
 
 # Do you want Talisman enabled?
-TALISMAN_ENABLED = True
+TALISMAN_ENABLED = False
 # If you want Talisman, how do you want it configured??
 TALISMAN_CONFIG = {
     'content_security_policy': None,