Allow for mapping to System Id in Excel and Database imports

User Story Template:

As a Solution Builder

I want The ability to map the unique identifier of my import file to the unique identifier of my target which is the System id. Rather than having to do all kinds of workarounds where I use an auto incrementing number, or export the System id and then save it into a string field on the Element to map to.

So That I can much more easily and consistently create export/import synchronizers that utilize the unique identifier already present in MooD for each Element.

Benefits 

1. Smaller footprint for the MooD database, currently anything you might want to import to at some point needs to have an additional field defined for that purpose, which results in a new Element data row for each element.
2. No more need to rely on an auto incrementing number series for imports, a number is fine for display purposes as it's easy enough for the user to read and understand, but they are not ideal for import purposes as you often end up with scenarios where you need to import data with elements of multiple types, and all of the sudden you have repeating id numbers.
3. Imports no longer rely on a value which can be modified by a MooD user
4. Easier to test your configuration and perform bulk imports in development, currently to avoid potential number series conflicts it's a bad idea to create any kind of elements in a repository you will merge into production, as there may have been new elements created in production in the interim, and as such you will end up with overlapping auto incrementing id numbers, or in the case of test elements, you might have a gap in the numbers. 
5. No need to maintain an additional string field on the element containing the system id value.

Acceptance Criteria I can map my import Unique Id to the System Id value for the given Element Type

Customer / Originator Solution builder

Priority High

PS. If this proves impossible, then perhaps the ability to add a new GUID field type that uses SQL newid() or something to create a different read-only system id that can be included in matrices and mapped to as unique identifier in imports.