Owner Added Email: It does not mention the right role for the new owner (when owner is a maintainer)

[etagwerker]

Steps to Reproduce

1. Add an owner
2. Set the new owner with the maintainer role
3. Get an email from Rubygems.org

Expected Behavior

I'd expect the email to use the right role (e.g. maintainer): "User was added as a maintainer..."

Current Behavior

I see that the email says that the new maintainer is added as an owner which causes unexpected confusion.

Possible Solution

Incorporate the role to the email that is being sent?

I see two potential solutions:

1. Tweak https://github.com/rubygems/rubygems.org/blob/master/app/views/owners_mailer/owner_added.html.erb to mention the role of the new owner
2. Generate a new mailer method (think owner_added vs. maintainer_added) the email template would be very similar

Environment

All browsers.

Additional Context

I believe the maintainer role was added here: #4883

Maybe it missed updating the email?