async fetchSignupSourcesWithRange(startDate, endDate) {
        const knex = this.knex;
        const startDateTime = moment.utc(startDate).startOf('day').format('YYYY-MM-DD HH:mm:ss');
        const endDateTime = moment.utc(endDate).endOf('day').format('YYYY-MM-DD HH:mm:ss');
        
        const rows = await knex('members_created_events')
            .select(knex.raw(`DATE(created_at) as date`))
            .select(knex.raw(`COUNT(*) as signups`))
            .select(knex.raw(`referrer_source as source`))
            .where('created_at', '>=', startDateTime)
            .where('created_at', '<=', endDateTime)
            .groupBy('date', 'referrer_source')
            .orderBy('date');

        return rows;
    }
Fetch signup sources with date range
@param {string} startDate 
@param {string} endDate 
@returns {Promise<SignupCountStatDate[]>}