Beginning of Strict CSP Compliance (#6841)

* Add CSP Support and enable it initially for Login page

* Update README

* Make sure that CSP is not enabled if CSP_POLICY isnt set

* Add a new value for x-csp header that tells if instance has opted-in to CSP or not

* Add more src to CSP

* Fix typo in header name

* Remove duplicate headers fn

* Add https://eu.ui-avatars.com/api/

* Add CSP_POLICY to env.example

.env.example

@@ -157,3 +157,6 @@ NEXT_PUBLIC_COMPANY_NAME="Cal.com, Inc."
 # Set this to true in to disable new signups
 # NEXT_PUBLIC_DISABLE_SIGNUP=true
 NEXT_PUBLIC_DISABLE_SIGNUP=
+
+# Content Security Policy
+CSP_POLICY=