Sort mailer previews alphabetically

[dwightwatson]

Summary

This fixes an issue where the index page of mailer previews was not sorted in any meaningful way. It appears as though the methods underneath each mailer are sorted alphabetically but the mailers themselves are not.

Now all the mailers will be ordered alphabetically along with their methods which is more predictable and expected.

Other Information

Here's a screenshot from my preview index as-is, where you can see the mailers are not in any obvious order.

[rails-bot]

Thanks for the pull request, and welcome! The Rails team is excited to review your changes, and you should hear from @sgrif (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

This repository is being automatically checked for code quality issues using Code Climate. You can see results for this analysis in the PR status below. Newly introduced issues should be fixed before a Pull Request is considered ready to review.

Please see the contribution instructions for more information.