[web] Group usernames case insensitively in MembersList

Summary:
Related Linear issue here.

Aboshcapital, aboshlocal, and Ashoatcapital's names all start with the letter A, but two groupings are created because the case is different. This diff merges the two groups together and groups member usernames case insensitively.

Test Plan:
Tested on Chrome/Safari and works as expected.

Before:

After:

Reviewers: tomek, atul, inka

Reviewed By: atul

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D5000