Hides the "Hide menu bar" option on MacOS (#2903)

The "Hide menu bar" option is only applicable to Windows and some Linux distros,
where the menu bar is attached to the Signal window. Therefore, this commit
ensures that it doesn't show up on MacOS. It includes a setting,
isHideMenuBarSupported(), to control the option's appearance. This
commit also includes the tests to make sure isHideMenuBarSupported()
works correctly.

Fixes #2705

settings.html

@@ -54,10 +54,12 @@
       </div>
     </div>
     <br />
+    {{ #isHideMenuBarSupported }}
     <div class='menu-bar-setting'>
       <input type='checkbox' name='hide-menu-bar' id='hide-menu-bar'/>
       <label for='hide-menu-bar'>{{ hideMenuBar }}</label>
     </div>
+    {{ /isHideMenuBarSupported }}
     <hr>
     <div class='notification-settings'>
       <h3>{{ notifications }}</h3>