Bug 22660: (follow-up) Improve asset handling, add linting

Bug 22660: (follow-up) Improve asset handling, add linting

This patch makes some changes to the way assets are included based on
the value of the system preferece in order to minimize unnecessary
JavaScript includes.

The patch also adds HTML syntax-highlighting and linting like we have in
the system preferences editor.

Removed are two JS files which were not required.

To test, apply the patch and set the NewsToolEditor preference to
"CodeMirror."

 - Edit a news item and confirm that the contents of the
   editor show HTML syntax highlighting.
 - Test HTML linting by adding some malformed HTML (missing closing tag,
   for instance). The error should be highlighted.
 - Set the NewsToolEditor to "TinyMCE" and confirm that WYSIWYG editing
   still works correctly.

https://bugs.koha-community.org/show_bug.cgi?id=22600

Signed-off-by: Lisette Scheer <lisetteslatah@gmail.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>