Cindy/flyer categories (#113)

* Implemented  getAllFlyerCategories and tests

* update jest

* update jest

* implemented suggestions from zach, vin, and archit

* got rid of ESLint comment

* fixed names for tests

* Added loops to test cases

* Fixed test case to create flyers with the same category

* Changed 'push' to 'concat'

* Updated `getAllFlyerCategories`

---------

Co-authored-by: Cindy <cl2329@cornell.edu>
Co-authored-by: Vin Bui <vdb23@cornell.edu>

src/repos/FlyerRepo.ts

@@ -76,7 +76,7 @@ const getFlyersByCategorySlug = async (
  * @returns a list of Strings with all the category names.
  */
 const getAllFlyerCategories = (): Promise<string[]> => {
- return FlyerModel.collection.distinct('categorySlug');
+ return FlyerModel.find().distinct('categorySlug').exec();
 };
 
 const getFlyerByID = async (id: string): Promise<Flyer> => {