Smarter `buildAboutPage` task

No issue
- Only run the buildAboutPage task if the `-contributors.hbs` template doesn't exist,
- ... or if the `--force` option is supplied on the CLI
- Adds the contributors img dir and the contributors template to `grunt clean:built` task
Merge pull request #3945 from novaugust/smart-contributors

Smarter `buildAboutPage` task