Looking at the MDN pages vs our latest/3.7 documentation, I see the following directives are deprecated and/or need some sorting out.

Rule	Docs	Note	Drop or keep?	Target release
`CSP_BLOCK_ALL_MIXED_CONTENT`	MDN docs	`Warning: This directive is marked as obsolete in the specification: all mixed content is now blocked if it can't be autoupgraded.`	Deprecate in 3.8 docs; remove in 3.10/4.0	3.8, 3.10/4.0
`CSP_PLUGIN_TYPES`	MDN docs	`This feature is non-standard and is not on a standards track. Do not use it on production sites facing the Web: it will not work for every user. There may also be large incompatibilities between implementations and the behavior may change in the future.`	Keep and add note to django-csp documentation	3.8
`CSP_PREFETCH_SRC`	MDN docs	As above	As above	3.8
`CSP_REPORT_URI`	MDN docs	`Warning: Though the report-to directive is intended to replace the deprecated report-uri directive, report-to isn't supported in most browsers yet. So for compatibility with current browsers while also adding forward compatibility when browsers get report-to support, you can specify both report-uri and report-to:`	Keep and update docs	3.8

Deprecated Features #197

Description