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
Remove suggestion that Procs can be used as session secrets.
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
Fix documentation to content_tag_for [ci skip]
Update some code examples on the "Working with JavaScript in Rails" guide.
Merge pull request #8771 from lucasmazza/unobtrusive-javascript-examples

New code examples on "Working with JavaScript in Rails" guide.
Remove the content_tag_for change from CHANGELOG since it was reverted
fix guides' warning bottom padding
Merge pull request #8772 from goshakkk/guides-warning-fix

Fix guides' warning bottom padding
prefer do-end for multiline block
Merge pull request #8774 from goshakkk/multiline-block-do-end

Prefer do-end for multiline block [ci skip]
Update mailer generator USAGE

* it does not generate fixtures
* it generates .text.erb templates
Fix generated migration file name in model generator USAGE
The controller generator invokes assets generator in addition