Add very basic oauth and mastodon api support.

lib/pleroma/plugs/oauth_plug.ex

@@ -0,0 +1,22 @@
+defmodule Pleroma.Plugs.OAuthPlug do
+  import Plug.Conn
+  alias Pleroma.User
+  alias Pleroma.Repo
+  alias Pleroma.Web.OAuth.Token
+
+  def init(options) do
+    options
+  end
+
+  def call(%{assigns: %{user: %User{}}} = conn, _), do: conn
+  def call(conn, opts) do
+    with ["Bearer " <> header] <- get_req_header(conn, "authorization"),
+         %Token{user_id: user_id} <- Repo.get_by(Token, token: header),
+         %User{} = user <- Repo.get(User, user_id) do
+      conn
+      |> assign(:user, user)
+    else
+      _ -> conn
+    end
+  end
+end