Revork mail template to display image/css

Summary

The application send mail on each status change of Incident/Maintenance. On particular mailbox application the theme is not displayed correctly.

Steps to reproduce

Activate the mail feature
Subscribe with and email
Create an incident/maintenance to trigger a mail notification

What is the current bug behavior?

On any mailbox application that doesn't load external resources, the mail appears as plain text

What is the expected correct behavior?

The mail should be displayed with the loaded CSS and if possible images (not too many)

Relevant logs and/or screenshots

Environment info

0.18.0

Possible fixes

Embded CSS and Images directly inside the mail instead of creating meta links

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information