Add helpers for mailer footer styles

[decabeza]

Objectives

This PR adds helpers for mailer footer styles as done in https://github.com/consul/consul/pull/4818.

[javierm]

Hi, @decabeza :smile:.

Not sure I'm following here :thinking:. Is this the same feature that was implemented in pull request #4301?

Sorry if I've misunderstood something! :pray:

[decabeza]

Hi, @decabeza 😄.

Not sure I'm following here 🤔. Is this the same feature that was implemented in pull request #4301?

Sorry if I've misunderstood something! 🙏

@javierm Oh! I was not aware of that PR! 😅 But that one looks much better! 👌

I'm not closing this one for the moment because I think I can modify it a bit and use the second commit to add the style helpers to the mailer footer. ☺️