Create categories for specific areas

https://trello.com/c/7dnoZOMf/4-as-a-user-i-can-browse-all-categories-within-a-specific-geographic-area
These conditions can be combined into a single line for clarity:

``` html
<% if signed_in? && current_user.admin? %>
  <%= render "categories/form", target: @category %>
<% end %>
```