Add end point to retreive before / after subjects based on their priority order

[camallen]

Linked to zooniverse/shakespeares_world#296

Add a new end point to support transcription page turning tools. This will find the subject based on the priority field and select the next N neighbours (up to a limit),
E.g /subjects/:id/sequential?limit=4 -> [ B4, B3, B2, B1, SUBJECT_ID, A1, A2, A3, A4 ]

This will have to copy the subject priority metadata field to the sms priority field. We can figure out the standard for this as we go along and manually do this for our projects that need this.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[camallen]

this keeps coming up for transcription projects, multi images subjects solve this to some degree but won't help with large numbers of pages.