Add mentioned users by status view (#28)

* Wip

* Add mentioned users menu to status

* Add test code

app/controllers/api/v1/statuses/mentioned_accounts_controller.rb

@@ -0,0 +1,74 @@
+# frozen_string_literal: true
+
+class Api::V1::Statuses::MentionedAccountsController < Api::BaseController
+  include Authorization
+
+  before_action -> { authorize_if_got_token! :read, :'read:accounts' }
+  before_action :set_status
+  after_action :insert_pagination_headers
+
+  def index
+    cache_if_unauthenticated!
+    @accounts = load_accounts
+    render json: @accounts, each_serializer: REST::AccountSerializer
+  end
+
+  private
+
+  def load_accounts
+    scope = default_accounts
+    scope = scope.where.not(id: current_account.excluded_from_timeline_account_ids) unless current_account.nil?
+    scope.merge(paginated_mentioned_users).to_a
+  end
+
+  def default_accounts
+    Account
+      .without_suspended
+      .includes(:mentions, :account_stat)
+      .references(:mentions)
+      .where(mentions: { status_id: @status.id })
+  end
+
+  def paginated_mentioned_users
+    Mention.paginate_by_max_id(
+      limit_param(DEFAULT_ACCOUNTS_LIMIT),
+      params[:max_id],
+      params[:since_id]
+    )
+  end
+
+  def insert_pagination_headers
+    set_pagination_headers(next_path, prev_path)
+  end
+
+  def next_path
+    api_v1_status_mentioned_by_index_url pagination_params(max_id: pagination_max_id) if records_continue?
+  end
+
+  def prev_path
+    api_v1_status_mentioned_by_index_url pagination_params(since_id: pagination_since_id) unless @accounts.empty?
+  end
+
+  def pagination_max_id
+    @accounts.last.mentions.last.id
+  end
+
+  def pagination_since_id
+    @accounts.first.mentions.first.id
+  end
+
+  def records_continue?
+    @accounts.size == limit_param(DEFAULT_ACCOUNTS_LIMIT)
+  end
+
+  def set_status
+    @status = Status.find(params[:status_id])
+    authorize @status, :show_mentioned_users?
+  rescue Mastodon::NotPermittedError
+    not_found
+  end
+
+  def pagination_params(core_params)
+    params.slice(:limit).permit(:limit).merge(core_params)
+  end
+end