refactor the naming within plugin new generator

Having a method called `full?`, which checks on :full and :mountable
is very confusing. I renamed `full?` to `engine?` and created a `full?`
method that only checks the `:full` option
Redirect using action hash instead of hardcoding full route
Move style to head to make routes page valid html5
Close container div tag in routing error page
Remove suggestion that Procs can be used as session secrets.
Remove warning, remove not used variable, and make methods private

Warning: "shadowing outer local variable - routes".
allow toggling about env, not just showing
Merge pull request #8768 from goshakkk/about-env-toggling

Allow toggling "about app's env", not just showing
config.assets.enabled is now true by default
Make sure assets is disable when --skip-sprockets is used
No need to give an example where there is generated code right there to serve as the example
deprecate `assert_blank` and `assert_present`.

They don't add any benefits over `assert object.blank?`
and `assert object.present?`
Merge pull request #8769 from senny/deprecate_assert_blank

deprecate `assert_blank` and `assert_present`.
fix img width in guides
Fix indention issue in generated template
Fix indention issue in generated template
Assert config.filter_parameters should be able to be set in a initializer
Needless fancy structure that did not anything
Merge pull request #8766 from jcoglan/session_token_docs

Remove suggestion that Procs can be used as session secrets.

[ci skip]
Move config.filter_parameters to you own initializer file

This is very rarely changed and does not deserve to be in application.rb