PLT-5080 (WebApp): Channel Admin permission level. (#5047)

* PLT-5080 (WebApp): Channel Admin permission level.

For channel management and channel deletion.

* Fix some incorrect showing/hiding of options.

* Remove incorrect client calls that was overwriting my channel members.

webapp/utils/constants.jsx

@@ -877,6 +877,7 @@ export const Constants = {
     LICENSE_EXPIRY_NOTIFICATION: 1000 * 60 * 60 * 24 * 15, // 15 days
     LICENSE_GRACE_PERIOD: 1000 * 60 * 60 * 24 * 15, // 15 days
     PERMISSIONS_ALL: 'all',
+    PERMISSIONS_CHANNEL_ADMIN: 'channel_admin',
     PERMISSIONS_TEAM_ADMIN: 'team_admin',
     PERMISSIONS_SYSTEM_ADMIN: 'system_admin',
     PERMISSIONS_DELETE_POST_ALL: 'all',