Update rep productivity definition

Related to https://gitlab.com/gitlab-com/finance/issues/866 and https://gitlab.com/gitlab-data/analytics/issues/2781

Why is this change being made?

@mbenza @wwright I am attempting to start the discussion on what the definition of rep productivity should be with this MR. Please tag whoever needs to know/agree to it. Based on whatever definition is agreed upon, next I will add the correct chart for the KPI.

The changes to the current definition that need to be reviewed are:

1. Currently we include IACV from opportunities owned by managers, directors, VP's, in our rep productivity calculation. This change would mean that rep productivity calculations only include IACV from opportunities owned by quota carrying sales reps.
2. Currently we include IACV from opportunities owned by Sales Admin (web portal purchases) as SMB IACV, and we divide out an extra SMB rep so that the real reps do not look more productive than they actually are. This change would mean that instead of including web portal purchases with SMB, we instead separate out web portal purchases into their own calculation.

cc @kathleentam