[web] Unify MenuItem casing

Summary:
MenuItems have different casing conventions. We have 'Promote to channel' and 'Leave Chat' next to each
other. Now we will have 'Leave chat' to be consisent with the rest of the web application.

Test Plan: Check on web if the MenuItems text has changed as expected.

Reviewers: ashoat, palys-swm

Reviewed By: ashoat, palys-swm

Subscribers: palys-swm, Adrian, atul, yayabosh

Differential Revision: https://phab.comm.dev/D4550