Debranding (wip, to keep track of changes so far)

[Meow]

WIP WIP WIP

This PR intends to make any aspects of Philomena deemed booru-specific (such as site name, description, and other parts of its branding, identity or unique functionality) configurable in a standardized manner.

Goal: for every booru to run the same stock image of Philomena

Highlights:

• Moves all .json configs to the database
• Adds configs which can be set either from the environment or from the database
  • If a config is set from the environment, it takes precedence over the database
• Changes all references to Derpibooru to references to "site_name" config, or similar configs
• Changes all pony-specific references to neutral franchise-agnostic references
• Moves any static assets which were brand-specific (e.g. favicon) to object storage
• Provides a way to set a tag as "invalid" which would prevent it from being added to images in a more systematic and customizable way as opposed to a .json file

ToDo:

• Add missing contexts, schemas, controllers and views
• Provide copyright-free franchise-agnostic images to the dev seeds
• Make commission categories configurable via database
• Implement config GUI
• Implement asset uploading GUI