Remove 'api plugin' from docs

AbstractController::Rendering is module which defines thie api
No need for .rb extension
Include AbsC::Rendering interface in ActionMailer
Move protected_instance_variables & view_assigns to AbstractController
Remove abstract_controller load hooks
Move anonymous class to the top, add documentation
Code formatting & typo fixes
Load AV::Layouts dynamicly via railties
Remove view_assigns from AV

It wasn't adding anything, just invoking super
Indent protected methods

[ci skip]
Remove unused in AV fixture-controllers
Remove unused ActionDispatch::RoutingVerbs
Remove ApplicationController & RoutingTestHelpers
Remove AV::TestCase from AP
Remove RenderERBUtils from AP (only AV use it)
Don not require AC::Caching
Do not silance mail gem warnings.

I doesn't have any at the moment
Load AV::Layout to AM::Base in railties
Include AV::Layouts before setting view_paths
Fix railtie tests