The thoughtbot style guide has changed since the code generated by `rails g
clearance:install` and `rails g clearance:specs` was written, and therefore
using the generators on a new project generates a lot of warnings from Hound
[1].

This commit fixes style guide violations in the generated code:

* Prefer double quotes to single quotes.
* Use `expect` instead of `should` in generated specs.
* Add missing newlines at the end of template files.

[1]: https://houndci.com/

* Rails infers reverse of `create_table`
* http://guides.rubyonrails.org/migrations.html#using-the-change-method

* Rename method to `sign_in`
* Put helper methods in alpha order