The Parent Project ID field in the proposals form should only accept valid parent projects

In virtually all cases, the only valid projects to list as a Parent Project are top-level projects.

We generally discourage arbitrary nesting of subprojects within subprojects, but there are a small number of cases where this makes sense. For example, we did at one point have an "Eclipse Woolsey" project that was a subproject of "Eclipse Dash" which is itself a subproject of the "Eclipse Technology" top-level project. We do not currently have a means of knowing which subprojects can be listed as the parent of other subprojects.

I'm thinking that it would be helpful to limit the acceptable values in the Parent Project field to those that make sense. Anything else should be flagged as an error.

Valid/permitted values are the union of the top-level projects and the designated "willing parent" subprojects.

I can envision adding a "willing parent" field -- accessible only by the EMO -- that can be used to toggle whether or not a subproject appears as an option for the Parent Project field.

Note that the Parent Project field also exists in the Project content type, but is only accessible to EMO. IMHO, we don't necessarily need to protect this field.

/cc @mdelgado624