Fix page buttons (#188)

* add prev/next grades selectors

* fix PageButtons props

src/data/selectors/grades.js

@@ -205,6 +205,8 @@ const simpleSelectors = simpleSelectorFactory(
     'filteredUsersCount',
     'totalUsersCount',
     'gradeFormat',
+    'nextPage',
+    'prevPage',
     'showSpinner',
     'gradeOverrideCurrentEarnedGradedOverride',
     'gradeOverrideHistoryError',

src/data/selectors/grades.test.js

@@ -287,6 +287,8 @@ describe('grades selectors', () => {
     testSimpleSelector('filteredUsersCount');
     testSimpleSelector('totalUsersCount');
     testSimpleSelector('gradeFormat');
+    testSimpleSelector('nextPage');
+    testSimpleSelector('prevPage');
     testSimpleSelector('showSpinner');
     testSimpleSelector('gradeOverrideCurrentEarnedGradedOverride');
     testSimpleSelector('gradeOverrideHistoryError');