Use a custom YAML coder to restore backwards-compatible deserialization of serialized query parameters

[cbeer]

Fixes #2768

[barmintor]

The Rails 6.1 failure is surprising - I just checked the Rails patch files and it looks like the config is exposed on ActiveRecord::Base.use_yaml_unsafe_load and not ActiveRecord.use_yaml_unsafe_load. Suggested a change.

[jrochkind]

Anything that would make it hard to backport to BL 7.x too?

[jrochkind]

Maybe a comment explaining why this is necessary, and that it becomes no longer necessary if in a Rails version including rails/rails#45591

(If that rails gets merged, as it looks like it will, then at some future point where BL only supports Rails versions where the latest patch includes that rails change, this custom coder will no longer be necessary)

Or the comment could be down with the custom coder class itself, maybe.

[barmintor]

Suggested one explanatory comment, otherwise I think this is good while Rails sorts itself out