Fix request #7373: Remove "target" settings for link in CKeditor configuration

With the default CKeditor, we can set the target attribute for links
(eg: <a href="…" target="_blank">…</a>). This is an issue because this
attribute is forbidden for HTMLPurifier and there is an inconsistency.

Now when user click on the [link] icon, she cannot anymore set the
target attribute. The same for [image > link].

Change-Id: If03519b4352bbffa681ceda52a10efc4395bdbb8
Merge commit 'refs/changes/24/2924/1' of ssh://gerrit.tuleap.net:29418/tuleap into HEAD

* ssh://gerrit.tuleap.net:29418/tuleap:
  Fix request #7373: Remove "target" settings for link in CKeditor configuration

Change-Id: If8394b49c680c045f4cab339ba35faeec778381b
This is Tuleap 7.4.99.3

Change-Id: Ife55190e07d07590bb421afaa22cfe69cb96ce32