Make configuration in local.js match config/mail.js

[DominusKelvin]

Since configs in locals.js gets merged into other configs, it makes more sense not to deviate from how mailers are being configured in config/mail.js. Currently we have:

mailer: 'nodemailer',
mailer: {
   nodemailer: {}
}

But we can however follow same configuration signature in config/mail.js and eliminate the checks in getting what mailer to use in production or development all together. With this change setting up an email mailer in local will look similar to that in config/mails.js.

mail: {
 default: 'nodemailer',
 mailers: {
    nodemailer: {}
}
}

This simplifies things.

• update code to reflect this decision
• update docs