Presave conversion rate for easy sorting by meta_value

[danieliser]

In the popup list table currently there is no way to easily sort by calculated conversion rate.

Related to: #775

[fpcorso]

@danieliser Is this now within Popup Analytics or is this something we still need with PM core?

[danieliser]

@fpcorso - So conversion tracking is not in core yet, so this is associated with that. If we do bring in a simple conversion counter to core, then yea we need this. Should probably associate the two issues.

As for if we should, with the rebrand, most likely should have something basic in the free version so they see how well it works.

[fpcorso]

@danieliser Ah, just hunted down #775 and added it to this issue's description. I'll continue the conversation of "should" over in that issue then.