Admins can create events, users can view events

**Admin actions and layout changes:**
* Create `AdminController`
* Add "Admin Dashboard" link to application layout for admin users
* Add `Guest` Null Object class
* Override `current_user` to return a Guest object when logged out
* Add "Home" link to application layout

**Admin users can create events:**
* Create model and migration for `events`
* Add namespaced routes for `admin/events#new` and `admin/events#create`
* Create `Admin::EventsController`
* Create view for `admin/events#new`
* Add `admin_dashboard` route to `events#new`

https://trello.com/c/amUeklmR

**Guests and users can view events:**
* Add routes for `events#show` and `events#index`
* Create `EventsController` (w/ `skip_before_action :require_login`)
* Create views for `events#show` and `events#index`

https://trello.com/c/JhJ32Jz4