Pass form or post/comment details through the editor

Sometimes you may need to access information contained elsewhere within a form inside the editor. For example, passing IDs into a REST API when uploading images. Currently there is no way to pass this information into the editor, and no way to know what form is the current editor.

It may be possible to pass an actual post ID through the editor, or the container/form the editor is within.

The issue names no files, tests, or entry points. Start by tracing how the editor is initialized inside post, comment, or form containers, then define which context should be exposed and how callers will use it; done requires an agreed interface and tests covering the supported contexts.

Written by the indexing model from the issue text.