Port dav calendar settings page to Vue.js

- Drop reliance on deprecated global jQuery object.
- Allow testing user interactions.
- Use newer technology stack.

---

Test user interactions with the groupware dav settings

Add infrastructure to test Vue components:

- Use recommended libraries:

    - https://vuejs.org/v2/guide/testing.html#Recommendations
    - Use jest-dom for robust assertions on the DOM state
    - Use user-event to be more representative of user actions

- Code is transpiled by Jest, with the help of vue-jest.

Ignore test files for no-unpublished-import. Prevent ESLint from
flagging:

```
/home/runner/work/server/server/apps/dav/src/views/CalDavSettings.spec.js
Error:   1:24  error  "@testing-library/vue" is not published         node/no-unpublished-import
Error:   2:23  error  "@testing-library/user-event" is not published  node/no-unpublished-import
```

Signed-off-by: François Freitag <mail@franek.fr>

Makefile

@@ -30,6 +30,7 @@ lint-fix-watch:
 clean:
 	rm -rf apps/accessibility/js/
 	rm -rf apps/comments/js/
+	rm -rf apps/dav/js/
 	rm -rf apps/files/js/dist/
 	rm -rf apps/files_sharing/js/dist/
 	rm -rf apps/files_trashbin/js/
@@ -46,6 +47,7 @@ clean:
 clean-git: clean
 	git checkout -- apps/accessibility/js/
 	git checkout -- apps/comments/js/
+	git checkout -- apps/dav/js/
 	git checkout -- apps/files/js/dist/
 	git checkout -- apps/files_sharing/js/dist/
 	git checkout -- apps/files_trashbin/js/