added /viewmylisting functionality w/o edit capabilities

[lenad90]

• Filter listings to user's id and populate each listing with the ability to delete a post. The list of listings is sent to the user by DM.

Files implemented: ViewMyListingCommand.java, ListingController.java

ViewMyListingCommand should populate and display listings for a specific user when they use the /viewmylisting command. Deletion of listing in the database when the user presses "delete" button after the listings are displayed.