Rename /invite_email quick action to /add_email

What does this MR do and why?

Related to #458407 (closed)

based on feedback in Investigate naming options for invite_email qui... (#458407 - closed) we should streamline the name of the external participants quick actions.

This MR changes the name of the /invite_email quick action to /add_email.

MR acceptance checklist

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. The feature flag issue_email_participants is enabled by default. If it's not on your system:

   Feature.enable(:issue_email_participants)

2. Create a new issue or browse to an existing issue.
3. Start to type the quick action name /add_email. You should see the autocomplete item for this quick action.
4. Write something like /add_email user@example.com and save the comment. This should add the email to the conversation (success message and system note).
5. Thanks for checking this MR locally 🤝