MAGN-1314: Hide Input & Output nodes in Home Workspace

The Issue
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Previously, one could see "Input" and "Output" nodes in search while in a Home Workspace.  This is bad, as these nodes have no meaning in this context and it should not be possible to place them.  

The Fix
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This PR removes that ability by adding two different kinds of Attributes to Dynamo: NotSearchableInCustomNodeWorkspace and NotSearchableInHomeWorkspace.  The latter is applied to both the Symbol (known to users as "Input") and Output types.

Unit tests
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This PR also revives three unit tests in SearchSideEffects.cs that tested for this functionality.  These tests were turned off for migration work hence the regression went unnoticed.

Further work
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This work made it obvious that SearchViewModel is due for a good redesign and separation into several components for handling various kinds of search.   This will be included in a separate pull request.