Add parameter for the spoken consent statement only when IRB requires a change

[mekline]

Is your feature request related to a problem? Please describe.
A lab's IRB is requesting a change to the spoken consent statement; they would like the consenting adult to say "I have read and have no further questions regarding the information the consent document describes. I am this child's parent or legal guardian and I agree to participate in this study and I agree to permit my child to participate."

There is already a pattern for parameters to the video-consent frame that are to be used only when a researcher's IRB requires a change from our usual template. We should add the text of the consent statement to this list!

Is your feature request related to a particular frame?
https://lookit.readthedocs.io/projects/frameplayer/en/latest/components/exp-lookit-video-consent/doc.html

Describe your idea or solution

If there's already a sensible name for this variable in the code, we should reuse it. Otherwise, add a new parameter to this frame called consent_statement_text. When not given, the value should continue to be the standard blurb ("I have read and understand the consent document. I am this child's parent or legal guardian and we both agree to participate in this study.")

[mekline]

I made a new issue for the documentation corresponding to the new parameter! lookit/lookit-docs#272