Per viewmodel ReadOnly mode

Some systems want to make use of distinction between opening a page for viewing - and intending to edit the data within.

The reasons for this may be many but one that was top of mind during our design is that we have inexperienced users that are afraid to touch or update things they do not intend to update. These users can be made to feel sa lot safer if there is a switch on the page you need to flip in order to edit anything.

Of course users still want to search and navigate the system - so a the readonly model should function much like AccessGroup ViewEnable expression - that only effect (locks down) actions and controls that actually change persistent data.

Resources and abilities available now for a framework backed per page readonly model:

1. vSysReadOnlyMode:Boolean : a Special ViewModel variable will be detected and applied
   1. When seen it will be "or"ed into the ViewModel.ExternalReadOnlyExpression - sharing this expression with any AccessGroups.ViewEnableExpressions.
2. WillEffectPersistedDataOverrideValue : a tagged value to be applied to columns and actions that you want to exempt from the automatic expression deduction if it changes persistent data or not.