Shortcodes: use has_shortcode to check for shortcodes in content

`has_shortcode` was introduced in WordPress 3.6 as a better way to check whether the content passed contains a specific shortcode ([ref](http://codex.wordpress.org/Function_Reference/has_shortcode)).

Since Jetpack supports WordPress current -1, I think it's safe to start using that function now.

This pull request updates all occurences where we were checking for shortcodes:
- In `Jetpack_PostImages`
- In the Contact Form builder
- In the Presentations shortcode