Add ClearSiteDataMiddleware

[Gee19]

Add a ClearSiteDataMiddleware and respective django settings.

CLEAR_SITE_DATA_URL_WHITELIST - whitelist of URLs that Clear-Site-Data response header is applied to (eg. /accounts/logout/)
CLEAR_SITE_DATA_DIRECTIVES - what directives to apply (defaults to wildcard)

https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Clear-Site-Data

[RouganStriker]

I think there is a doc or README we need to update to document the new middlewre

[Gee19]

I think there is a doc or README we need to update to document the new middlewre

Looks like docs are autogenerated based on index.rst

[lmontopo]

This MR and the other look good to me. I haven't tested it, but I can if you'd like me to. CR +1.

[RouganStriker]

I think there is a doc or README we need to update to document the new middlewre

Looks like docs are autogenerated based on index.rst

I don't think the README file gets auto-updated though; I just remember having to manually update that every time I modify a middleware.

[Gee19]

@RouganStriker Oh right! Good call I forgot about the list of middlewares there

[lmontopo]

+1.