Make HTML and text emails for new issues uniform and add the mail to mailer previews

What does this MR do?

Changes the HTML template to always show the author of the issue (independent of the email_author_in_body setting) as by https://gitlab.com/gitlab-org/gitlab-ce/issues/58065#note_147551622

Also added the new_issue_email to the Mailer Previews (https://docs.gitlab.com/ee/development/emails.html#mailer-previews)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Closes #58065 (closed)