Finish adding authentication to pixtr

* Implement sessions#destroy
* Add sign out link to layout
* Add monban gem to Gemfile
* Add password_digest field to users
* Update Controllers to use monban
* Update sessions#new and users#new to have a password field
* Create homes#show
* Extract sign in and sign out forms to partials
* Add before_action on Galleries
* Add a has_many relationship between User and Gallery
* Add a list of galleries to Users#show