Model Documentation updates
Add text to summary of SetAttributeValue that informs developer that it does not persist values to database
Fix saving of Global Attributes so that it immediately updates the cached global attribute values

fixes #120
Add PreSelected flag to check-in objects so that actions can use them to preselect certain options that blocks can then use
FilterGroupsByAbilityLevel: Fixed invalid attribute comparison
FilterGroupsByGender: Fixed action to apply to all groups, not just the selected group
LoadGroups: Removed old LoadAllLocations attribute
SaveAttendance: Set default code length that was already being set in code but not applied
Rock.csproj: Added SelectByBestFit and SelectByLastAttended
ExtensionMethods: Added ParseNullable<type?> and GetDescription(enum)
Removed comment in ExtensionMethods & added a clear description to SelectByLastAttended.
Merge branch 'feature-ds-updateworkflowactions'
Merge branch 'develop'
Model Documentation updates