Don't filter annotations when showing "All Students"

When the client is running in the LMS app and in grading mode, don't
apply a broken filter to the annotations list when the LMS app is asking
the client to show "All Students".

Fixes https://github.com/hypothesis/lms/issues/1259

src/sidebar/store/modules/selection.js

@@ -169,16 +169,26 @@ const update = {
   },
 
   CHANGE_FOCUS_MODE_USER: function(state, action) {
-    return {
-      focusMode: {
-        ...state.focusMode,
-        enabled: true,
-        focused: true,
-        config: {
-          user: { ...action.user },
+    if (action.user.username === undefined) {
+      return {
+        focusMode: {
+          ...state.focusMode,
+          enabled: false,
+          focused: false,
         },
-      },
-    };
+      };
+    } else {
+      return {
+        focusMode: {
+          ...state.focusMode,
+          enabled: true,
+          focused: true,
+          config: {
+            user: { ...action.user },
+          },
+        },
+      };
+    }
   },
 
   SET_FORCE_VISIBLE: function(state, action) {

src/sidebar/store/modules/test/selection-test.js

@@ -213,6 +213,23 @@ describe('sidebar/store/modules/selection', () => {
       assert.equal(store.focusModeFocused(), true);
       assert.equal(store.focusModeEnabled(), true);
     });
+
+    // When the LMS app wants the client to disable focus mode it sends a
+    // changeFocusModeUser() RPC call with {username: undefined, displayName:
+    // undefined}:
+    //
+    // https://github.com/hypothesis/lms/blob/d6b88fd7e375a4b23899117556b3e39cfe18986b/lms/static/scripts/frontend_apps/components/LMSGrader.js#L46
+    //
+    // This is the LMS app's way of asking the client to disable focus mode.
+    it('disables focus mode if username is undefined', function() {
+      store.setFocusModeFocused(true);
+      store.changeFocusModeUser({
+        username: undefined,
+        displayName: undefined,
+      });
+      assert.equal(store.focusModeFocused(), false);
+      assert.equal(store.focusModeEnabled(), false);
+    });
   });
 
   describe('setFocusModeFocused()', function() {