Tweaked the new blog post tags feature

[javiereguiluz]

This continues #447 and introduces some minor changes.

---

The only relevant changes for end users is the minor redesign of the tags. For example, in the blog index, before tags stand out too much and they look like buttons:

Now they are faded because, after all, they are a secondary information:

In the blog post show, the tags are bigger:

And in the admin, I haven't modified anything, so they still look like this:

[yceruto]

👍

Travis fails with https://travis-ci.org/symfony/symfony-demo/jobs/198778450#L320:

1) Tests\AppBundle\Controller\Admin\BlogControllerTest::testBackendHomepage
The backend homepage displays all the available posts.
Failed asserting that actual size 31 matches expected size 30.

Did you added another post manually?

[yceruto]

CS expects an endpoint here, maybe should be reverted :/

[javiereguiluz]

Once we update the PHPCsFixer to 2.0, we can disable that rule because : should be valid as the ending of a phrase.