Bugfix recipients of design comment notifications

What does this MR do?

Previously, a user would only receive an email notification from a comment on a Design if they were mentioned in the comment. This MR changes the scopes of users who can receive an email notification to (from #217095 (comment 439656335)):

• All users who uploaded versions of the design
• All users who have commented on the design
• All users who have been mentioned in comments on the design

(As well as the usual user and project notification settings that apply.)

• Issue: #217095 (closed)

QA

A basic QA would be:

• Have user 1 comment on a design.
• Have user 2 also comment on a design (without mentioning user 1).
• User 1 should receive an email notification.

The other scenarios in #217095 (comment 439656335) can also be QA'd.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Related to #217095 (closed)