feat(graphql): validate timezone id as a GraphQL Scalar

Related to #1299

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

lib/graphql/schema/custom/timezone.ex

@@ -0,0 +1,35 @@
+defmodule Mobilizon.GraphQL.Schema.Custom.Timezone do
+  @moduledoc """
+  The timezone scalar type allows timezone ID strings to be passed in and out.
+  """
+  use Absinthe.Schema.Notation
+  import Mobilizon.Web.Gettext, only: [dgettext: 3]
+
+  scalar :timezone, name: "Timezone" do
+    description("""
+    The `Timezone` scalar type represents a timezone identifier,
+    as registered in the IANA Time Zone Database.
+    """)
+
+    serialize(&encode/1)
+    parse(&decode/1)
+  end
+
+  @spec decode(Absinthe.Blueprint.Input.String.t()) :: {:ok, term} | :error
+  @spec decode(Absinthe.Blueprint.Input.Null.t()) :: {:ok, nil}
+  defp decode(%Absinthe.Blueprint.Input.String{value: value}) do
+    if Tzdata.zone_exists?(value),
+      do: {:ok, value},
+      else: {:error, dgettext("errors", "Timezone ID %{timezone} is invalid", timezone: value)}
+  end
+
+  defp decode(%Absinthe.Blueprint.Input.Null{}) do
+    {:ok, nil}
+  end
+
+  defp decode(_) do
+    :error
+  end
+
+  defp encode(value), do: value
+end