Add Teaser plugin and some other goodies

Big pull request ... sorry for that.

This add an basic implementation of the Teaser plugin, but working (with this pending patch applied in some case (https://review.typo3.org/23279). This plugin can be improved but work nicely. Currently the path of the posts storage node, is in Settings.yaml. In the futur this can be configured in TS or directly in the plugin inspector.

Other goodie, Thumbnail viewhelper, extract the first image in the blog post and use it has a thumbnail in blog post list.

French translated XLIFF