Fix bulk send

[jeffm2001]

Fixes # (issue)

Description

Fixes a GraphQL error when using the bulk send feature. In the bulkSendMessages mutation, it was looking for assignmentId as an int, but receiving it as a string.

Possibly related:
#2237
#2492

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