E-mails: Drop usage of campaign-mailgun, use SMTP instead

[tiblu]

Overview

campaign-mailgun uses Mailgun API, but Mailgun can be used over SMTP so it provides no extra value.
Rather, it provides extra problems - the Campaign switches on e-mail tracking which rewrites links in the e-mails.

TODO

• Drop usage of campaign-mailgun
• Configure servers to use campaign over SMTP

[ilmartyrk]

Still relevant. campaing-mailgun also has some deprecated packages and functions too just to make this task more relevant

[ilmartyrk]

possible library to use https://github.com/mailgun/mailgun.js

[anettlinno]

Triage 37. Current campaign-mailgun is outdated and needs to be updated to what Ilmar proposed above.
Approx. dev. time 1 day. Sending to development.

[ilmartyrk]

Update campaign and related packages, created pull requests. All in all it is really good library. Fixed all related packages too. Hopefully pull requests are accepted and we can revert back to getting packages from npm. But if that will not happen in recent month or so will create our own release or merge this directly to our code.

[ilmartyrk]

Added test coverage for all our current e-mails but these would need to be more thorough, created a new issue for that as it is not with so high priority #229