email: use name_with_namespace for email subjects

What does this MR do?

Adds the namespace to the project section of email subjects.

Are there points in the code the reviewer needs to double check?

I don't think so.

Why was this MR needed?

Filtering on

What are the relevant issue numbers?

N/A

Screenshots (if relevant)

N/A

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together