Send email when a token or co-owner is added

[jsha]

In the spirit of #1895, it would be good to notify account holders and crate owners on actions that affect authorization status, like adding an API token to their account or adding a co-owner to a crate they own.

[domodwyer]

Hey @Turbo87 I'm happy to finish this ticket off (by adding the "send an email when a co-owner is added" functionality).

Before I do, in #9245 (comment) you said:

we could move sending the email to a background job, but that would be yet another query. this is fine as is for now :)

Would you prefer I implement the new co-owner email as a background job? Or inline with the API call like #9245 does for the token email. If background job, I'll open a PR after to do the same for #9245 too after?

[Turbo87]

Would you prefer I implement the new co-owner email as a background job?

whatever you prefer/is easier. we don't have strong reasons yet to prefer one over the other.