Add anonymous and remote participations

lib/web/email/checker.ex

@@ -10,4 +10,19 @@ defmodule Mobilizon.Web.Email.Checker do
   """
   @spec valid?(String.t()) :: boolean
   def valid?(email), do: email =~ @email_regex
+
+  @spec validate_changeset(Ecto.Changeset.t(), atom()) :: Ecto.Changeset.t()
+  def validate_changeset(%Ecto.Changeset{} = changeset, key \\ :email) do
+    changeset = Ecto.Changeset.validate_length(changeset, :email, min: 3, max: 250)
+
+    case Ecto.Changeset.fetch_change(changeset, key) do
+      {:ok, email} ->
+        if valid?(email),
+          do: changeset,
+          else: Ecto.Changeset.add_error(changeset, :email, "Email doesn't fit required format")
+
+      :error ->
+        changeset
+    end
+  end
 end