Bundle: email sending on invitation/adding a domain manager

[abroddrick]

Issue description

We have several email tickets for sending emails when someone is invited to a domain (via the registrar or admin) and when adding an existing member as a domain manager.

With these tickets we will switch to a dynamic email, so that if a user is invited/ added as a domain manager to more than one domain then content of email changes. We will also add in email sending on the members Domain assignment page and send emails to all domain managers if someone is invited/ added as a domain manager to a domain.

Acceptance criteria

• Dev: Make updates to domain invitation emails #2960
• Add email sending to the portfolio domain assignment review page #3295
• Dev: Send emails when someone is added as a domain manager #2370

Additional context

This tickets can be done in separate PRs, the same PR or any combination

note: there is a follow on ticket for removing the domain manager, this is a great candidate to pick up next after these but was not included in the bundle given there was so more conversation to be had with removing a manager.

Links to other issues

• relates to Send email if a domain manager is removed from a domain #3285