[WIP] Insert Pages links, Document Files links and external links to pages

This is WIP. Tasks to complete:

* Add styles
* See the edit link and find the page by the site label and the slug
@dougdroper and @finitud with this structure you can have the page mirrors for any page. I used in the page list dialog:

![screen shot 2014-12-05 at 11 54 49](https://cloud.githubusercontent.com/assets/27509/5315469/a9fa949e-7c75-11e4-8c7f-6694911f2807.png)

As I talked with @dougdroper  I tried to not put many 'welsh' and 'english' code related (with the exception of inspect because help you a lot to understand the object itself).

You can play with that class in the console and will join the page with the corresponding mirrors:

```
[1] pry(main)> pages = Comfy::Cms::Page.all; nil
=> nil
[2] pry(main)> PageMirror.collect(pages)
=> [#<PageMirror english_label: 'Foo Bar', english_url: '/en/foo-bar', welsh_label: 'Cy Welsh bar', welsh_url: '/cy/foo-bar'>]
```