Replace Fancybox with Magnific-Popup

This only switches the plugin, doesn't make any other code changes.
Removed unused template snippets

The unused functionality was removed some time ago in 371d7bcf6.
Replaced Fancybox functionality with Magnific-Popup

Fixes bug 2929.
UNDO    API: Add sample pootle-cli client for API

It is meant for learning purposes
UNDO  not working tests with pootle-cli