Added an option to only display first x number of photos, but still be able to show them with next/previous, not just the current page.

- added migrations due to an extra field
- added an option to show all photos - just select "all" at per page setting
- added an option "photos_to_show" to only display a certain number of images
- added total number of photos to the context
- these options allow you to only show 3 photos, but show all of them with previous/next when clicking on one of them (you have to tweak the album_plugin.html)