Now that RelatedObjectedWidget is in Djblets, we import

it from there and extend it in the RelatedUserWidget.
Created RelatedRepositoryWidget and RelatedGroupWidget, along with

relevant HTML and JS files, which extends the RelatedObjectsWidget.

RelatedGroupWidget has the ability to only display groups that are

invite only. All three widgets, for exampe, can be viewed on

/admin/db/reviews/defaultreviewer/add/, and they all function.
See /r/10214.

Created relatedUserSelectorTests.es6.js, 

relatedRepoSelectorTests.es6.js, and 

relatedGroupSelectorTests.es6.js to test the widgets 

javascript functionality.
Ran ./tests/runtests.py and no errors were thrown. Ran the js-tests, and

no errors were thrown.
New unit tests were created in reviewboard.admin.tests that test

the functionality of the widgets. The test classes are:

RelatedUserWidgetTestCase, RelatedRepositoryWidgetTestCase and,

RelatedGroupWidgetTestCase.
Navigated to /admin/db/reviews/defaultreviewer/add/ and the 

RelatedUserWidget, RelatedRepositoryWidget and 

RelatedGroupWidget function as intended.