Support force dropping database introduced in PostgreSQL 13 (#280)

This is introduced in PostgreSQL 13 (https://www.postgresql.org/docs/current/sql-dropdatabase.html)
and will be enabled when `:force_drop` option is enabled.

lib/ecto/adapters/postgres.ex

@@ -68,6 +68,8 @@ defmodule Ecto.Adapters.Postgres do
     * `:lc_collate` - the collation order
     * `:lc_ctype` - the character classification
     * `:dump_path` - where to place dumped structures
+    * `:force_drop` - force the database to be dropped even
+      if it has connections to it (requires PostgreSQL 13+)
 
   ### After connect callback
 
@@ -147,13 +149,15 @@ defmodule Ecto.Adapters.Postgres do
     end
   end
 
-  defp concat_if(content, nil, _fun),  do: content
+  defp concat_if(content, nil, _),  do: content
+  defp concat_if(content, false, _),  do: content
   defp concat_if(content, value, fun), do: content <> " " <> fun.(value)
 
   @impl true
   def storage_down(opts) do
     database = Keyword.fetch!(opts, :database) || raise ":database is nil in repository configuration"
-    command  = "DROP DATABASE \"#{database}\""
+    command = "DROP DATABASE \"#{database}\""
+              |> concat_if(opts[:force_drop], fn _ -> "WITH (FORCE)" end)
     maintenance_database = Keyword.get(opts, :maintenance_database, @default_maintenance_database)
     opts = Keyword.put(opts, :database, maintenance_database)