Code editor: replace CodeMirror with ACE in the explanation text

Adjust the explanation text for the new code editor.

Signed-off-by: xchs <github@gmx.net>
Support creating primary indexes on multiple columns in the DCA
Merge pull request #5593 from xchs/hotfix/xpl-code-editor
Merge the custom layout sections upon theme import (see #5000)
Mark the active versions in the versions overview
Show a preview image when editing image meta data (see #4948)