Stats: show email open/click rate instead of absolute numbers

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We show absolute numbers for email open/clicks, which is already helpful:

Typically products show these numbers as rate compared to number of sent emails, which is a variable that changes over time. That makes it much easier to see how open/click rate is developing over time, and it's easier to grok if rate is actually high or low for particular issue.

Rate is basically how publishers are used to see these numbers.

Example from MailerLite:

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