This adds the functionality of replacing all images that were uploaded
to gitlab with inline images(base64) in emails.
This change fixes the broken images in emails that 7.8 introduced
Rationale:
* In this notification email, the message is the most important content.
It should not be dimmed or smaller than the rest of the text.
* Now that all UI elements are removed, the message *is* the email. It
should be written as a standard email message, without styling.
