Stop needMessageCount input from zeroing-out

[oburbank]

Fixes # (issue)

https://github.com/MoveOnOrg/Spoke/issues/1587

Description

I added React Formal's mapToValue method to override the value retrieved by name, which can become stale as new texters are added.

Checklist:

• I have manually tested my changes on desktop and mobile
• The test suite passes locally with my changes
• If my change is a UI change, I have attached a screenshot to the description section of this pull request
• My change is 300 lines of code or less, or has a documented reason in the description why it’s longer
• I have made any necessary changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• My PR is labeled [WIP] if it is in progress

[schuyler1d]

Thank you! Needed to tweak this a little bit to:
mapToValue={m => m.texters.find(t => t.id === texter.id).assignment.needsMessageCount} to make the text input update properly.