feat(slugs): allow filters in template strings

Process template filters first, then strip `'` and `.` characters from
slug strings.

Fixes #4783

packages/netlify-cms-core/src/lib/__tests__/formatters.spec.js

@@ -274,7 +274,7 @@ describe('formatters', () => {
   };
 
   describe('slugFormatter', () => {
-    const date = new Date('2020-01-01');
+    const date = new Date('2020-01-01T13:28:27.679Z');
     jest.spyOn(global, 'Date').mockImplementation(() => date);
 
     const { selectIdentifier } = require('../../reducers/collections');
@@ -312,6 +312,18 @@ describe('formatters', () => {
       ).toBe('entry-slug');
     });
 
+    it('should allow filters in slug templates', () => {
+      selectIdentifier.mockReturnValueOnce('published');
+
+      expect(
+        slugFormatter(
+          Map({ slug: "{{published | date('MM-DD')}}" }),
+          Map({ title: 'Post Title', published: date }),
+          slugConfig,
+        ),
+      ).toBe('01-01');
+    });
+
     it('should return slug', () => {
       selectIdentifier.mockReturnValueOnce('title');
 

packages/netlify-cms-lib-widgets/src/stringTemplate.ts

@@ -190,13 +190,13 @@ export function compileStringTemplate(
         replacement = data.getIn(keyToPathArray(key), '') as string;
       }
 
+      const filterFunction = getFilterFunction(filter);
+      if (filterFunction) {
+        replacement = filterFunction(replacement);
+      }
+
       if (processor) {
-        return processor(replacement);
-      } else {
-        const filterFunction = getFilterFunction(filter);
-        if (filterFunction) {
-          replacement = filterFunction(replacement);
-        }
+        replacement = processor(replacement);
       }
 
       return replacement;