adds senders to database and firebase connect

- senders, which now have a firebase token
  are added as their own table in the database
- firebase token api call added, which allows
  a user to connect a sender id to a firebase token
- firebase library not yet connected to the saved
  firebase tokens

priv/repo/migrations/20211230132931_add_senders.exs

@@ -0,0 +1,13 @@
+defmodule Bump.Repo.Migrations.AddSenders do
+  use Ecto.Migration
+
+  def change do
+    create table :senders do
+
+      add :sender, :string, size: 255, null: false
+      add :firebase_token, :string, size: 2048 
+
+      timestamps([type: :utc_datetime])
+    end
+  end
+end