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