This just removes the background.ts code that detects whether the user
wants to clobber their csp and adds an error message if the user tries
to `set csp clobber`.
The `csp` setting is marked as deprecated but left untouched in case we
find a way to edit CSP in a way that complies with Mozilla's policies.
The csp-editing code in `requests.ts` is left untouched for the same
reason.
As discussed on riot, turning noiframeon into a site-specific setting
is a good thing for consistency. This commit also adds a deprecation
warning when users try to `:set noiframeon` and updates the
troubleshooting guide.
