[MM-67123] fix: change the user at_mention button 'type' attribute to 'button' to avoid triggering message submit

webapp/channels/src/components/profile_popover/profile_popover_controller.tsx

@@ -128,6 +128,7 @@ export function ProfilePopoverController<TriggerComponentType = HTMLSpanElement>
                 ref={refs.setReference}
                 className={props.triggerComponentClass}
                 style={props.triggerComponentStyle}
+                type={TriggerComponent === 'button' ? 'button' : undefined}
                 {...getReferenceProps()}
             >
                 {props.children}