Fix/gh152 fix firefox print preview artifacts alsp add supporters shortcode

[gin0115]

Changes proposed in this Pull Request

• Adds in a css rule to ensure the background with clipping is hidden when print preview is loaded to avoid Z index issues
• Adds in shortcode rendering for supporters

Testing instructions

BEFORE

AFTER

A list of supporters can be added to any post using the [display_supporter]

This will display all of the supporters in a simple list.

You can also pass a limit=10 if you only want to show the first 10 (or any count) of supporters.

Suporters are shown alpabetically

Mentions #152