Update interactions.md to reflect the recent refactoring.
Add placeholder spec in anticipation of the new JobsAppliedToReport and JobsAppliedToReportGenerator classes.
Add complete spec for the basic String output of JobsAppliedToReportGenerator.
Make Posting#add public in anticipation of the new report spec.

Add PostingListAppender module, with #add_posting_to_postinglist, in anticipation of the new spec.
Add #jobapplications_submitted_by, #postings_submitted_to_by_jobseeker, and #jobs_submitted_to_by_jobseeker to JobApplicationRecordList in anticipation of the new spec.

Have JobApplicationRecordList#jobapplications_submitted_for_posting use List#each instead of List#select, as it discards the List generated by #select.
Build out JobsAppliedToReport and JobsAppliedToReportGenerator classes, fulfilling the new spec.