New beauty pages(404, 422, 500)

moving changelog note to unreleased section

New beauty pages(404, 422, 500)
Remove hard coded references to Active Record in railties
Merge pull request #9512 from Jacke/master

New 404, 422, 500 pages, they are more stylish and bring ruby essence…
integrate the strong params README into the AC guide.

The current ActionController guide does not mention strong parameters
at all. I integrated the README into the guide to explain the API.

I also included a section to illustrate that the API does not solve
all possible whitelisting scenarios.

The origin was #9454.
Merge pull request #9465 from senny/document_strong_params_in_ac_guide

integrate the strong params README into the AC guide.
fix the sql that is generated from scoping
Use custom visitor class for optimized url helpers

Rather than trying to use gsub to remove the optional route segments,
which will fail with nested optional segments, use a custom visitor
class that returns a empty string for group nodes.

Closes #9524
Bring config.allow_concurrency back

Since the Rack::Lock still exists in development,
let's provide a way to disable it explicitly.
Created an unscope method for removing relations from a chain of
relations. Specific where values can be unscoped, and the unscope method
still works when relations are merged or combined.
Merge pull request #9532 from neerajdotname/fix-sql-in-example

In example fix sql that is generated from scoping
Merge pull request #9200 from wangjohn/unscoping_activerecord_merging

Introduce relation #unscope