Add poster: parameter to Helpers::posted_job.

Add optional poster: parameter to Helpers::posted_job, which takes an instance of JobPoster.
Add job posting list report spec.

Add "Employers should be able to see a listing of the jobs they have posted" spec.
Fulfill posted job reporting spec.

Fulfill spec confirming that employers can get a list of jobs that they have posted, to the express exclusion of jobs that they have not posted even if they are in the list.
Add employer-specific job posting report spec.

Add spec checking whether employers can get a list of the jobs they have posted, to the exclusion of jobs posted by others.
Remove unused method.

Remove PostedJobList#report, as it is not actually being used yet.
Fulfill employer-specific job posting report spec.

Fulfill spec confirming that employers can get a list of the jobs they have posted, to the exclusion of jobs posted by others.
Actually use PostedJobList.

Refactor EmployersPostedJobReportGenerator to actually use PostedJobList to filter for only posted jobs instead of doing so itself.

Update specs accordingly, removing the extraneous creation of PostedJobLists.

Update PostedJobList to actually filter by poster.