Slug category and tag url

[clement-michelet]

Using tags/categories with spaces and special characters (like with accents), it will generate unfriendly url for category/tag page.

Examples :

Tag static site generator will generate a page url /blog/tags/static%20site%20generator/ which is a little bit unfriendly.
By using slug, it will generate a page url /blog/tags/static-site-generator/ which is clean and clearer.

Tag révolution technologique will generate a page url /blog/tags/révolution%20technologique/ which is still not friendly and could lead to bugs because of the special chars.
By using slug, it will generate a page url /blog/tags/revolution-technologique/ which is bug free.

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We can leverage new String component to ease slugification. If Twig is upgrade to 3.x, it will also be possible to generate those url with the same logics.