[web/native] Small changes to the change roles stack
Summary:
There were three suggestions left across the change-roles stack, and they were all small enough that I thought it made sense to just cover it all in one diff. The three changes are:
- Use shorthand for createChangeThreadMemberRolesPromise
- Remove a <>...</> that doesn't do anything
- Move the activity indicator styles down to unboundStyles
Test Plan: Confirmed that everything still works as expected
Reviewers: atul, ashoat
Reviewed By: ashoat
Subscribers: tomek
Differential Revision: https://phab.comm.dev/D8378