* Unset X-Frame-Options header correctly
According to the [spec](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Frame-Options) there are onlye the `DENY` and `SAMEORIGIN` options, `ALLOW-FROM` being deprecated. Hence we have to actively unset the header, as we made `DENY` the default.
This also unsets the X-Frame-Options header for the public form pages, which fixes#4666.
* Ignore anti forgery token in Forms
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Co-authored-by: nicolas.dorier <nicolas.dorier@gmail.com>