Move common Content functions to a new helper

Developers have to reuse the content of the `ControllerBlockContent` to correctly handle the HTML code.
Let's move this common code to a new content helper.

For the underlying discussion: see https://www.concrete5.org/developers/pro-accounts/community-leaders-area/community-leaders-discussion/content-block-link-translation/

PS: I also fixed the indentation. For a cleaner view of what's changed see https://github.com/concrete5/concrete5/pull/1696/files?w=1