Future Hydrus (self-hosted *booru) support

[Bill]

Homepage: http://hydrusnetwork.github.io/hydrus/
Repo: https://github.com/hydrusnetwork/hydrus
Scripts: https://github.com/CuddleBear92/Hydrus-Presets-and-Scripts
Community contacts: https://discord.gg/vy8CUB4

1. Draft a standard API for Hydrus developer to build (API is on to-do list)
2. Adapt such API to Zenbooru