Prevent framing on sites

[mvl22]

This article just published recommends that web devs in future should not permitting framing of sites:
https://www.theregister.com/2021/03/08/post_spectre_programming/
using the header called X-Frame-Options.

There's no reason to allow this, now that the white label sites have been split out of the codebase.

This should be added to various Apache configs.