Add "Part of" Research Study extension and Search Parameter to Profiles

[RobertJCarroll]

This continues the work discussed in #55 and #153.

At a high level, linking data back to the study to which it belongs is a common and critical task for users of FHIR data/APIs. To accomplish this, we have implemented an extension that enables implementers to reference a study (#153). However, this remains an "invisible" option in the IG as it does not appear on Profile pages. Additionally, it will not be searchable without additional search parameters.

To complete this issue, we need:

1. Extension added to "all applicable" Profiles in the IG.
2. "Improved documentation" about the extension and its use.
3. Search parameter(s) added to "all applicable" Profiles in the IG.

We still need to scope an explicit list of "all applicable" Profiles and if it is required. I anticipate this will be required for all "row-level" Profiles, but may not be required for some others. For example, Data Dictionaries may be shared across ResearchStudies, which may make complications.