Better permission checks on assignment-related mutations

[schuyler1d]

This makes a couple of plumbing changes:

1. consolidating around assignmentRequiredOrAdminRole and removing assignmentRequired.
2. removes unnecessary messageId querying now that there is no longer a maintained assignment_id in message table
3. Fixes Feature Request: Super volunteers should be able to use message review #1488

Description

This cleans up some permission checks and plumbing. It's in-service of more upcoming assignment-related mutations (e.g. releaseAssignment for https://github.com/MoveOnOrg/Spoke/issues/1533) and also explicitly checks for TEXTER role in service of https://github.com/MoveOnOrg/Spoke/pull/1562 (cc: @JeremyParker )

Checklist:

• I have manually tested my changes on desktop and mobile
• The test suite passes locally with my changes
• [na] 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
• [na] I have made any necessary changes to the documentation
• I have added tests that prove my fix is effective or that my feature works