Add translations for `session_login`, `session_logout`, and `register` endpoints

[dantownsend]

A simple dictionary which the user can define, which maps the English word / phrase to their local language.

For example:

# For Portuguese users:
register(translations={'register': 'registro'})

The only option at the moment is to create your own template. This is definitely easier.

We could probably include a few translations out of the box for some language (just pre-defined dictionaries):

from piccolo_api.shared.auth.translations import register_portuguese

# For Portuguese users:
register(translations=register_portuguese)