Adjust scheduling week to start on Sunday

[domdomegg]

Currently, the tool schedules cohorts beginning on Monday.

However, we want to schedule cohorts starting Sunday. This means people considering switching cohorts have two weekends to do a cohort in, which creates more flexibility. Without this, if people are away for weekend breaks and busy during the weekdays, they would have to completely miss a week.

We can currently work around this by manually editing the start dates of Sunday cohorts to a week earlier. But it'd be better to just do this directly in this extension.

BlueDot Internal Slack: https://bluedotimpact.slack.com/archives/C042KE94C78/p1723660804744029