Merge branch 'fix_email_images' into 'master'

Fix email images

## Dependencies:
This MR Depends on gitlab-org/html-pipeline-gitlab!4 being **merged and published** to rubygems.org

## What does this MR do?
This MR fixes the broken images in emails that occured scince we introduced access control for all attachments in 7.8.
The access control lead to broken images, because the user usually isn't logged into gitlab when opening the email in his mail client.
The solution to fix this, is to replace all images that were uploaded to gitlab as attachemnts with inline images in emails.

I only added one test for images in notes, because all notes share the same view. If it works fine for a note on a commit, then it'll also work the same for notes on a MR or on issues.

@DouweM can you review this please?

Closes #1161

See merge request !373