CRM-15166 crmCaseType - Tighten control over "Standard Timeline" and "Open Case"

These changes encourage the admin to preserve a few elements of the default
CaseType definition - but the GUI should still be able to open/save unusual
CaseTypes.

Also, it's possible to override things if you really work at it. For
example, if you unlock and rename "standard_timeline", then you can delete
"Open Case" or delete the entire activity-set.  Of course, you take your
fate into your own hands when you unlock that.)