Bug 21502: Add check for YAML formatted system preferences to about page

Bug 21502: Add check for YAML formatted system preferences to about page

So far there are these yaml formatted preferences:
    UpdateNotForLoanStatusOnCheckin
    OpacHiddenItems
    BibtexExportAdditionalFields
    RisExportAdditionalFields
    UpdateItemWhenLostFromHoldList
    MarcFieldsToOrder
    MarcItemFieldsToOrder

Test plan:
1) Add some badly formatted YAML into some of preferences listed above
2) Check the System information tab on About page for appropriate warnings

Signed-off-by: Michal Denar <black23@gmail.com>
Signed-off-by: Michal Denar <black23@gmail.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>