Extract mail configuration into a gem

[jhawthorn]

I feel that much like paperclip settings (#4139), mail settings should be configured only in initializers using the normal rails methods. (using config.action_mailer.smtp_settings)

Here I've extracted the functionality into a gem
https://github.com/jhawthorn/spree_mail_settings

Also will partially fix #3833

[radar]

I'm 👍 on this. Less code for us all to maintain.

[jhawthorn]

🚢 Finished up the gem and added a changelog entry. This should be good to go.

[GeekOnCoffee]

Love it, thanks for the work @jhawthorn!

Couple of potential thoughts:

• Should this get forked into the Spree organization?
• Can we put really harsh deprecation errors into place for this? My personal opinion is we should stop an app from starting if any of the removed configs are set, giving them good instructions on how to set it properly and clear up the error... There was a ton of confusion on the S3 settings that I'd like to avoid repeating

[FineLineAutomation]

+1 on the harsh deprecation errors.

[jhawthorn]

Forking to spree (or possibly the to-be-formed spree-community) sounds like a good idea.

I can't think of a decent way for it to give deprecation warnings for people who had previously set this in the admin but not an initializer. My hope is that stating the change clearly in the release notes and migration guide will help. This was not done for the S3 settings change in spree 2.1

[JDutil]

I think going to https://github.com/spree-community would be fine, which has been established. @jhawthorn your now added to it as well if you would like to get it setup there before this is merged.

[huoxito]

Showing 22 changed files with 13 additions and 525 deletions.

Awesome!

[jhawthorn]

I believe this is good to go. Repo has been moved to spree-contrib/spree_mail_settings

[radar]

@jhawthorn Apologies on the delay. I've added this to my master branch now.