Transaction updates

@cmcilhenny @daveyk535 @timsanty 

please review and merge!

- moved current_user methods into a SessionsHelper (was having trouble access the current user's id)
- added a redirect on the users index path so the nav bar can link to a single url and redirect to the correct one
- added seller method to product function to easily find who the seller is
- added links to user's transaction history to navbar
- added routes and set up buttons to deploy methods updating transactions when buyer clicks "buy" or seller clicks "accept"/"decline"
- updated seed data to remove avatars since they weren't working