Export participants to different formats

* CSV
* PDF (requires Python dependency `weasyprint`)
* ODS (requires Python dependency `pyexcel_ods3`)

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

priv/repo/migrations/20211001101902_create_exports.exs

@@ -0,0 +1,18 @@
+defmodule Mobilizon.Repo.Migrations.CreateExports do
+  use Ecto.Migration
+
+  def change do
+    create table(:exports) do
+      add(:file_path, :string)
+      add(:file_size, :integer)
+      add(:file_name, :string)
+      add(:type, :string)
+      add(:reference, :string)
+      add(:format, :string)
+
+      timestamps()
+    end
+
+    create(unique_index(:exports, [:file_path]))
+  end
+end