add "opportunity to improve - peer average" message template

[zachll]

Motivation: Adding the "opportunity to improve - peer average" would increase the value of the message collection for the pilot because the message uses the social worse pathway and creates an alternative message to "you are not a top performer" for cases where the recipient performance level is below average.

Solution: Add the message template to the mpog_pilot branch for testing

Notes: This will require the ability to select messages based on the lowest comparator level instead of gap size when there are multiple negative gaps, such as to the peer average and the 90th percentile benchmark

[pboisver]

Added in mpog pilot and merged to main