Enable setting of security http headers

Enable setting of security http headers

The following security policy headers are set:

X-XSS-Protection: 1; mode=block -- always set

X-Frame-Options: DENY is set for all pages except for the documentation
  pages, primarily because pgadmin4 loads them in an iframe which would
  break.

Content-Security-Policy: <x>-src
  Is set to allow the default of self only, then allowing scripts for
  google analytics and fonts for google fonts. Images are allowed from everywhere.
  frame-ancestors 'none' is set by the same rules as X-Frame-Options

This also adds a decorator for @script_sources to have a single view
allow extra sources, and this is used for recaptcha. A generic decorator
is also made for other types of exclusions, though we don't have any at
this point.

If the setting SECURITY_POLICY_REPORT_ONLY is set to True then the policy
will be report-only and not enforced (for testing), otherwise enforcing
mode is enabled.

The setting SECURITY_POLICY_REPORT_URI sets where to send security
policy reports, if any.