Example keywords validation (#2956)

This pull request closes #2551: making sure that the keywords don't already appear in the example's title.

I also added validation that checks that they are at least two characters long, as this is enforced by the homepage when type your search.

services/wordpress/wordpress-version.service.js

@@ -49,7 +49,6 @@ function VersionForExtensionType(extensionType) {
           title: `Wordpress ${capt} Version`,
           namedParams: { slug: exampleSlug },
           staticPreview: this.render({ response: { version: 2.5 } }),
-          keywords: ['wordpress'],
         },
       ]
     }