Make new issue, merge request and epic email have the same format

What does this MR do?

The issue, merge request, and epic new notifications have very similar initial lines, but they are each a little different.

This makes them consistent across all three, in the format:

Firstname Lastname created an issue: #1234

Firstname Lastname created a merge request: !1234

Firstname Lastname created an epic: &1234

This also makes the plain text version in the same format. Also internationalizes strings, as started in !56064 (merged)

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• 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

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

Related to #328338 (closed)