Add Show Sidebar toggle to Subscriptions view options

Adds a Show Sidebar toggle (iPad regular and macOS) that controls
channels sidebar visibility. When the sidebar is shown, the channel
strip picker is disabled and the redundant channel header link above
the feed is hidden. Layout picker now uses inline menu style for
consistency with other options.

Yattee/Localizable.xcstrings

@@ -17720,6 +17720,16 @@
         }
       }
     },
+    "viewOptions.showSidebar" : {
+      "localizations" : {
+        "en" : {
+          "stringUnit" : {
+            "state" : "translated",
+            "value" : "Show Sidebar"
+          }
+        }
+      }
+    },
     "viewOptions.layout" : {
       "localizations" : {
         "en" : {