INT-1763: Updating auto-casting of numbers.

[durran]

Compass no longer uses the Javascript type Number for numbers in the documents, it now uses the correct BSON type for serialization, either Int32, Int64, or Double.

This pull also fixes the auto-casting for the numbers. The following behaviour now happens:

1. When an Int32 is edited to a value over 32bits, the type automatically casts to Int64.
2. When an Int64 is edited to a value over 64bits, the type automatically casts to String.
3. When an Int32 is edited to a value over 32bits, the types drop-down contains both Int64 and Double until the value passes (+/-) Number.MAX_SAFE_INTEGER, then the drop-down only contains Int64 as a numeric value to cast to.

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This change is