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

### The Issue

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

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.  SearchViewModel checks for this attribute and hides or reveals the nodes when appropriate.

### Unit tests

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

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.  

@ikeough, @Benglin 
I'd go with ```private set``` in the following property, but oh well, doesn't matter for now:

```cs
public WorkspaceModel Workspace { get; private set; }
```