Bug 35078: Remove invalid HTML from OpacShowSavings system preference

Bug 35078: Remove invalid HTML from OpacShowSavings system preference

The options defined for the OpacShowSavings system preference contain
HTML: "on patron's checkout history page (the system preference
<em>opacreadinghistory</em> must be enabled)". This text ends up in an
<option> tag, and even though the form is rendered using the
multiple-select plugin the HTML still won't render and should
be removed.

To test, apply the patch and restart all.

- Go to Administration -> System preferences and search for
  "OpacShowSavings"
- Check the dropdown of options on the preference. The system
  preference names referred to should now be single-quoted instead of
  wrapped in visible <em> tags.
- Confirm that the preference still works correctly.

Signed-off-by: Caroline Cyr La Rose <caroline.cyr-la-rose@inlibro.com>
Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>