discord: add initial slash command integration.

Add initial support for the `/count <tags>` and `/posts <tags>` slash commands.

Slash commands are basically like webhooks; we register a command, and
when anybody types that command in Discord, Discord sends us a HTTP
request that we respond to.

* https://discord.com/developers/docs/interactions/slash-commands
* https://support.discord.com/hc/en-us/articles/1500000368501-Slash-Commands-FAQ

app/logical/discord_slash_command/count_command.rb

@@ -0,0 +1,28 @@
+class DiscordSlashCommand
+  class CountCommand < DiscordSlashCommand
+    def name
+      "count"
+    end
+
+    def description
+      "Do a tag search and return the number of results"
+    end
+
+    def options
+      [{
+        name: "tags",
+        description: "The tags to search",
+        required: true,
+        type: ApplicationCommandOptionType::String
+      }]
+    end
+
+    def call
+      tags = params[:tags]
+      query = PostQueryBuilder.new(tags, User.anonymous).normalized_query
+      count = query.fast_count(estimate_count: true, skip_cache: true)
+
+      respond_with("`#{tags}`: #{count} posts")
+    end
+  end
+end

app/logical/discord_slash_command/posts_command.rb

@@ -0,0 +1,93 @@
+class DiscordSlashCommand
+  class PostsCommand < DiscordSlashCommand
+    extend Memoist
+
+    def name
+      "posts"
+    end
+
+    def description
+      "Do a tag search"
+    end
+
+    def options
+      [{
+        name: "tags",
+        description: "The tags to search",
+        required: true,
+        type: ApplicationCommandOptionType::String
+      }]
+    end
+
+    def call
+      tags = params[:tags]
+      query = PostQueryBuilder.new(tags, User.anonymous).normalized_query
+
+      limit = query.find_metatag(:limit) || 3
+      limit = limit.to_i.clamp(1, 10)
+      posts = query.build.paginate(1, limit: limit)
+      embeds = posts.map { |post| post_embed(post) }
+
+      respond_with(embeds: embeds)
+    end
+
+    def post_embed(post)
+      {
+        title: post.dtext_shortlink,
+        url: Routes.url_for(post),
+        timestamp: post.created_at.iso8601,
+        color: post_embed_color(post),
+        footer: post_embed_footer(post),
+        image: {
+          width: post.image_width,
+          height: post.image_height,
+          url: post_embed_image(post),
+        },
+      }
+    end
+
+    def post_embed_image(post, blur: 50)
+      if is_censored?(post)
+        nil
+      elsif post.file_ext.match?(/jpe?g|png|gif/)
+        post.file_url
+      else
+        post.preview_file_url
+      end
+    end
+
+    def post_embed_color(post)
+      if post.is_flagged?
+        0xC41C19
+      elsif post.is_pending?
+        0x0000FF
+      elsif post.parent_id.present?
+        0xC0C000
+      elsif post.has_active_children?
+        0x00FF00
+      elsif post.is_deleted?
+        0xFFFFFF
+      else
+        nil
+      end
+    end
+
+    def post_embed_footer(post)
+      dimensions = "#{post.image_width}x#{post.image_height}"
+      file_size = post.file_size.to_s(:human_size, precision: 4)
+      text = "Rating: #{post.rating.upcase} | #{dimensions} (#{file_size} #{post.file_ext})"
+
+      { text: text }
+    end
+
+    def is_censored?(post)
+      post.rating != "s" && !is_nsfw_channel?
+    end
+
+    def is_nsfw_channel?
+      discord.get_channel(data[:channel_id]).fetch("nsfw")
+    end
+
+    memoize :is_nsfw_channel?
+  end
+end