Email instance view can show inconsistent open stats

[cjg89]

Describe the bug

Email instance views can occasionally show inconsistent open counts on a single page load, due to {{instance.initial_opens}} being re-evaluated each time it's called.

PostMaster/templates/manager/email-instance.html

Lines 56 to 63 in 2b97e8a

	<strong class="lead font-weight-black">{{instance.initial_opens}}</strong>
	</div>
	<div class="col-4 d-flex flex-column justify-content-between">
	<h4 class="font-weight-bold small text-uppercase">Open Rate</h4>
	<strong class="lead font-weight-black text-info-aw">{{instance.open_rate}}%</strong>
	</div>
	</div>
	<p class="small pt-3 mb-3">Of the <strong>{{instance.recipient_details.count}}</strong> recipients, <strong >{{instance.initial_opens}}</strong> opened the email. An open rate of <strong>{{instance.open_rate}}%</strong>.</p>

{{instance.click_recipient_count}} likely has the same issue.

Steps to Reproduce

1. Open an instance view of an email that's recently sent to a lot of people
2. Refresh until something like this happens
   

Expected Behavior

Stats like this should be consistent wherever they're printed on the page.

Possible Fix

Look into caching/storing property values like this to prevent them from being looked up multiple times on load. lru caching, custom template variables, {% with %} usage could be worth looking into

Your environment:

n/a

Additional context

n/a