Modify all the aggregation pipelines to sort the sections by section number, and then term

Modify all the **Trends-related** aggregation pipelines in the uploaders so that the list of sections can be sorted by section number first, and then the term (```section_number``` & ```academic_sesstion.name``` in section objects). 

Also move all the pipelines under a single file ```trends_pipeline.go``` since each pipeline is not that long or complicated to justify the use of separate files. 