Move root answer creating and reviewing into Mosaic

[brachbach]

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Something like this process: https://docs.google.com/spreadsheets/d/1ReZG66AHfe5lFXiBmIoVPL3wzzRXS9MdZA786N95HQM/edit#gid=1689997668

The advantages of moving it into Mosaic would be:

1. More convenient for participants -- all they have to do to get their next thing to work on is to go to their active experiment in Mosaic, rather than having to think about whether they should be creating/reviewing answers on the sheet or participating in a Mosaic experiment
2. Quicker to get these root answers into experiments -- rather than using the API to create trees with these answers, the trees will simply continue from the root answers created in Mosaic as soon as they're created
3. Experiments team no longer has to maintain the spreadsheet

Some things that'd be nice to have if we moved it into Mosaic:

1. Some way to have a participant who's not currently an expert on the tree create/review the answer, and then get assigned as an ME on the tree (note that anyone who creates or reviews an answer should get assigned as an ME, because they now know both root answers, which is context that only the ME should have)
2. Integrate all of the elements of the current answer creation survey: https://docs.google.com/forms/d/e/1FAIpQLSfRG6yq9EVAYlS6zpBY9g1MoRopTDeCYV4_mi1f6mxaFSqKxg/viewform