Updating required phrasing of all scheduled studies

Right now, we ask (in admin review) that all scheduled studies say

- This study will take place on a video call, live with a researcher! Clicking on the “Participate Now!” button will send you to an online calendar where you can select a date and time that works for you.

But this is out of date! The button says "Schedule a time to participate".

There are three things that should likely happen at the same time:

1. Update the admin review template (Mark can do this). The text about the button just needs to be updated.
2. Update all of the currently-posted live studies (we could tell researchers we intend to do this to their studies, with a downtime of just seconds per study, and then someone--likely Mark--could do a pass through all studies to make the change.
3. The [setting study details page on readthedocs](https://lookit.readthedocs.io/en/develop/researchers-set-study-fields.html) should be updated to indicate the required wording under "Short description"