Add setting to enable/disable HTML emails

What does this MR do?

It adds a new global config setting where the admin can enable/disable the usage of HTML emails.

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

Why was this MR needed?

Basically customers with a large number of projects and users want to save some disk space on their mail servers by only using plain text emails.

Screenshots (if relevant)

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 merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #24880 (closed)