Feature request: Pool order and pool browsing implementation

[404Fox]

This is a request for the rest of pools implementation. It doesn't seem there are any forks working on this right now.

See #321 for the initial PR adding pools. Quotes come from that post.

Pool order is not searchable/enforced

The order the posts are added in is saved to the database but it isn't used yet. The pool:id named token makes no assumptions about sorting, and it seems like the sort token feature only works on columns, so it seems difficult to make a sort:pool:id token since a single post belonging to many pools can be ordered differently depending on the pool searched for.

I don't know about the relationship of post-pool tables, but I'd assume the post doesn't need to be aware of its position in a pool? The order of the posts should be derived from the pool.

Pool order cannot be modified in GUI

Draggable thumbnail GUI on the pool page. Or instead of dragging, left/right buttons attached to each thumb.

Browse pools by thumbnails (of the first, or an assigned "cover" post)

Pools page but with thumbnails in a grid format. The first post in the pool can be the assigned cover. Also include the name of the pool in the grid member.

Mass pooling like mass tagging

I'm not sure if it's best to add it as a separate mass-tagging toggle like the one for tags already, or if there should be a "metatag" system like Danbooru 2 where posts tagged like pool:1234 automatically get assigned to pool 1234. That way the mass tagging feature already implemented can also be used to add posts to pools easily.

Post shows pool info; Browsing a pool from /post page -> See #354

• However, because a post can belong to multiple pools, the user must select the pool to browse it. This can be done using the pool list. When the user selects one, the main pool bar is displayed and starts browsing the pool pages. Or, pool browsing is only started from a pool:id query.
• Pool paging by passing the query, eg. /post/99/query=pool:7 and selecting the next post relative to the current post ID. Assuming pool:id returns the sorted results by default.

Whitespace for pool names

Currently it uses the same validation as tags, and the names are split on whitespace.