Enable customization of card template

[laggron42]

Backgrounds and image generator should be customizable through the admin panel and allow unique configurations for each regime and special event. #17 must be resolved as we want shiny balls to be included in this new logic.

The customizable options should be:

• Base size of the canvas
• Background image
• Font family, size and color
• Custom font family upload
• Layout and positioning of elements
• Expose properties from the extra data of models

I think these settings should be set using YAML format, allowing to inherit from previous definitions. The admin panel could expose those settings for easy edition.

[dormieriancitizen]

PR (not yet ready)