move send_file and send_from_directory to Werkzeug

The implementations were moved to Werkzeug, Flask's functions become
wrappers around Werkzeug to pass some Flask-specific values.

cache_timeout is renamed to max_age. SEND_FILE_MAX_AGE_DEFAULT,
app.send_file_max_age_default, and app.get_send_file_max_age defaults
to None. This tells the browser to use conditional requests rather than
a 12 hour cache.

attachment_filename is renamed to download_name, and is always sent if
a name is known.

Deprecate helpers.safe_join in favor of werkzeug.utils.safe_join.

Removed most of the send_file tests, they're tested in Werkzeug.

In the file upload example, renamed the uploaded_file view to
download_file to avoid a common source of confusion.

tests/test_regression.py

@@ -3,7 +3,6 @@ import platform
 import threading
 
 import pytest
-from werkzeug.exceptions import NotFound
 
 import flask
 
@@ -56,13 +55,6 @@ def test_memory_consumption():
             fire()
 
 
-def test_safe_join_toplevel_pardir():
-    from flask.helpers import safe_join
-
-    with pytest.raises(NotFound):
-        safe_join("/foo", "..")
-
-
 def test_aborting(app):
     class Foo(Exception):
         whatever = 42