Tags with metrics

[Hunternif]

This is a feature I implemented in my fork of szurubooru. Any tag can have a number associated with it. Examples: number of characters in the picture, "lightness" of a picture. Would this be useful as a PR to the main project? :)

Additional features include:

• any existing tag can have one metric
• posts can be queried and sorted by the value of a metric, with an exact or a range query
• a metric value on a post itself can be exact or a range
• the post page displays the metric value if present
• the post editor has a UI to set either the range or the exact value
• a separate "Metric sorter" UI where you determine the exact value by doing a "manual quicksort". This is for non-integer metrics, when it's hard to put an exact value on a new post without comparing it to reference images. The page shows pictures with known values one at a time, and you compare your picture to them, gradually narrowing down the range of your value.

The metric is defined in this new tab on the tag page:

The weird "manual quicksort" UI 😅. You click on the "<" button to change the comparison. On mobile this UI is vertical. The example is contrived and doesn't make sense for an integer metric, but you can imagine using it for something more abstract like "power" or "lightness".