Improved Gelbooru upload

Reconstruct the Gelbooru upload page with a bunch of useful features.

This extension reconstructs the Gelbooru image upload page with multiple features that make posting images as convenient and fast as possible. Interface - Uses nicely formatted tags instead of plain text - Displays a large preview of the uploaded image on the side - Makes it possible to upload multiple pictures in different tabs Tagging - Shows warning if an entered tag likely contains a typo or is non-standard - Type multi-word tags more easily using spaces instead of underscores - Built-in artist search for quickly adding artist tags given their URL - Option to split the tag input into custom groups to get a better overview over entered tags and facilitate a more systematic approach to tagging - Change the type of a tag (e.g. to artist/character) with just two clicks Image upload - Easily upload images by dragging them from other webpages - Automatically fills in the source URL when uploading images from Pixiv - Automatically checks if a picture is already uploaded on Gelbooru: 1. Starts with a fast MD5 hash check to find posts with the same image 2. For images from Pixiv, it conducts a source check using the Pixiv ID 3. If the searches above didn't find anything, it tries to find posts with similar images via reverse image search in the IQDB database