Implement video shortcode self hosted

[SergioEstevao]

Fixes https://github.com/wordpress-mobile/WordPress-iOS/issues/3911

This is basically a copy of the solution implemented in Android here: https://github.com/wordpress-mobile/WordPress-Android/commit/edded6ff8bbfdd36fdd2160c3c07094226aac80a

How to test:

• Start the demo app
• Tap to insert a video
• Wait for the upload to finish
• Switch to HTML
• Check if video was inserted using the [video] shortcode

[SergioEstevao]

@diegoreymendez if you want to have a look on the main app, I have test branch there called issue/3911_use_video_shortecode_on_self_hosted