Show slack dropdown consistently

[SpennyNDaJets]

Summary

The slack @ dropdown does not consistently render to the correct place. Drop the parent ref, and let react mentions render the dropdown under the input.

https://www.loom.com/share/ca0f989351a5480097812917614f8b0e

How did you test this change?

1. View a session
2. Click to add a comment
3. Type in the @ symbol

• Dropdown is displayed under the comment input

Are there any deployment considerations?

N/A

Does this work require review from our design team?

N/A

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Latest commit: e9cabec

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