[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