Fix front-end, allow events to be created by a group, allow to get sessions from an event

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

priv/repo/migrations/20180110092500_create_groups.exs

@@ -0,0 +1,19 @@
+defmodule Eventos.Repo.Migrations.CreateGroups do
+  use Ecto.Migration
+
+  def change do
+    create table(:groups) do
+      add :title, :string, null: false
+      add :slug, :string, null: false
+      add :description, :string
+      add :suspended, :boolean, default: false, null: false
+      add :url, :string
+      add :uri, :string
+
+      timestamps()
+    end
+
+    create unique_index(:groups, [:slug])
+
+  end
+end