Topic Subscription emails should include unsubscription link

[dd32]

Presently it appears that being subscribed to a topic doesn't offer a way to unsubscribe:

[...]
Post Link: https://wordpress.org/support/topic/example-topic/#post-1234
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You are receiving this email because you subscribed to a forum topic.

Log in and visit the topic to reply to the topic or unsubscribe from these emails. Note that replying to this email has no effect.

These emails should include an unsubscription link and header, similar to Tag subscription emails.