Background & Problem
As part of T294460: When filtering the mentees' table the last choice of filters should be saved, preference growthexperiments-mentee-overview-filters was created as a JSON blob storing filter configuration for the mentees API. However, it doesn't have a field to store page size in. The patch to persist pagesize introduces growthexperiments-mentee-overview-presets as a JSON blob to store all mentee overview-related presets, not just the filters (or just the page size).
To keep the number of properties the lowest, let's merge those two together.
This is blocked on r767177 to be merged.