Better defaults for the Show page

I'll turn this into a PR once there's code to go along with the idea.

This replaces the stale #1077.

```ruby
# Renders the default table
show do
  attributes_table
end

# Removes specific attributes
show do
  attributes_table except: [:foo, :bar]
end

# Adds extra (virtual) attributes
show do
  attributes_table with: :baz
end
```