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
Remove unnecessary begin..rescue..end, use only rescue
Update generated app's README [ci skip]

* a controller that destroying Weblogs might be WeblogsController
* a controller that indexing Posts might be PostsController
* Post.all returns a Relation
* modernize Model.inspect results
Include the standard reminder that you need to restart for changes in config/initializers to take effect
This is not that common of a change to warrant top-line billing and you should read more about the consequences than just toggling a boolean to make best use of it