add status method-raise level to will change when there are diffs

[arichardet]

A new method on status RaiseLevelForDiffs checks if there are any diffs and raises the level to StatusWillChange.
Resources were updated to call this new method in the following cases:

• The same check was being performed prior to raising the level to StatusWillChange
• Multiple calls to AddDifference/RaiseLevel to StatusWillChange: the calls to RaiseLevel were removed and a call to this new method was added at the end of the function.

Implements #586

[rebeccaskinner]

Can you also add a line into resource-authors-guide.md talking about the new function? LGTM otherwise.

[rebeccaskinner]

LGTM