refactor(middleware): Refactor internals of CSPMiddleware so that it's easier to extend existing logic without copy/pasting it into subclass

[crbunney]

The current implementation of csp.middleware.CSPMiddleware doesn't lend itself to extending the functionality of it or the csp.contrib.rate_limiting.RateLimitedCSPMiddleware without copy/pasting the implementation of .build_policy() and .build_policy_ro()

This is a bit of an issue, because it means any updates to the base implementation of the superclass has to be manually copied into any subclasses.

For example, for my multi-tenant webapp project, we use the RateLimitedCSPMiddleware and have an additional requirement to generate a CSP that contains some per-tenant configuration. So I've created a custom middleware by copying the code from RateLimitedCSPMiddleware and adding my extra logic in.

This isn't great because if the implementation of RateLimitedCSPMiddleware changes, for example to fix this issue #231, then I have to remember to check for changes and manually copy them into my middleware.

So, I'm proposing a change to how CSPMiddleware works to provide a better hook into the existing implementation and allow subclasses to modify things before the policy string is built.

As you can see from the diff, this reduces the lines of code required to create a CSPMiddleware subclass, even more so when extending the RateLimitedCSPMiddleware.

The build_policy and build_policy_ro methods are essentially redundant with this PR, but I wasn't too sure about removing outright, since it would a breaking change, so would like the maintainers' input on that.

Finally, I haven't attempted to address any of the known issues already raised in other issues/PRs

[robhudson]

I like where this is going. I have also noticed the copy/paste needed to extend the middleware. Thanks for submitting a patch.

I would support leaving build_policy and build_policy_ro for now, as you've done, and full removal in 4.1.0. But I would also like some documentation added to the migration docs on this change.

[crbunney]

I would also like some documentation added to the migration docs on this change.

I've added a section to the migration guide now

[robhudson]

This is great! Thank you for working through the requested changes.