Add a "latest changes" button and page

This is providing a more complete "latest changes overview", based on bartkamphorst first attempt in #822.

the second `gsub` removes file extensions, breaking the links to non-page files. Is it possible (and adequate) to just replace all unrecognized file extensions with something like...
```ruby
page_path = f.gsub(/^.*=> /, '').gsub(/\.(?!#{Gollum::Markup.formats.values.join("|")}).*$/, '')
```