Improve copy when user is already unsubscribed from a thread

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Steps to reproduce

• Subscribe to an issue/MR/thread on GitLab.
• When the thread has an update, an email notification would be sent to the email on file.
• Click on Unsubscribe at the bottom of the email. You should be unsubscribed from this thread.
• Click on Unsubscribe again.

What happens currently

A notice that says You have been unsubscribed from this thread. appears.

What could be improved here

When the user is already unsubscribed from that thread, it would be helpful to modify the messaging to indicate that the user has been already unsubscribed from this thread.

This is mostly an enhancement to the current behaviour, but if this feels to be an unnecessary improved, feel free to close the issue!