Set locale based on the content item

When displaying a translated content item, we want the locale to be set
accordingly. This ensures that we get the appropriate translated text in the
view template.
Use one global source for page direction

Now that we are overriding the global I18n.locale based on the loaded content
item, we can use a single, global helper to determine page direction in all
templates.
Remove the text_direction method from presenter

This is no longer being used as we are relying on the global page_text_direction
method to determine when content should be rendered right-to-left.
Make controller action clearer

The before_action calls were harming the comprehensibility of the code.
Merge pull request #11 from alphagov/set-locale-based-on-content

Set and override global locale