fixed error in get_message under django 2.2+

[murray3k]

urlsafe_base64_encode changed in django 2.2 so we get an exception:
.../env/lib/python3.5/site-packages/shared_session/templatetags/shared_session.py", line 58, in get_message return urlsafe_base64_encode(enc_payload).decode('ascii') AttributeError: 'str' object has no attribute 'decode'

See Django Docs:

urlsafe_base64_encode(s):
Encodes a bytestring to a base64 string for use in URLs, stripping any trailing equal signs.
Changed in Django 2.2:
In older versions, it returns a bytestring instead of a string.

I'm using Django's force_text to fix this issue and keep it backward compatible. Seems to work, tested it in 2.1.8 and 2.2.