Bug 23315: (follow-up) Add configuration to enable CodeMirror

Bug 23315: (follow-up) Add configuration to enable CodeMirror

This patch adds the "syntax" parameter to the corrected system
preferences so that the textareas will be displayed as CodeMirror
editors.

To test, apply the patch and test the affected system preferences:

Circulation -> SelfCheckHelpMessage
Staff client -> IntranetmainUserblock
Staff client -> IntranetCirculationHomeHTML
Staff client -> IntranetReportsHomeHTML
Staff client -> StaffLoginInstructions

With UseWYSIWYGinSystemPreferences disabled, the preferences should be
displayed as CodeMirror editors. With  UseWYSIWYGinSystemPreferences
enabled they should be WYSIWYG editors.

Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
(cherry picked from commit 02c9246def1f19bdc8310c562ddaf5172cf0b3ad)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>