Make text consistent: "Add new content" -> "Add content"

There's an inconsistency on the front page of newly installed Drupal 8 sites. A link appears twice with different texts: "Add new content" and "Add content" (see graphic). A search through core shows that this inconsistency appears in several other places as well.

I propose making it consistent throughout core. My preference is for "Add content" because it's shorter and the word "new" is unnecessary.

A patch follows.