Add task to create default user on database

[jfcalvo]

This PR adds a new task to create user with defaults credentials on database.

# Get help
$ python -m argilla.tasks.users.create_default --help

# Create the default user
$ python -m argilla.tasks.users.create_default

# Create the default user without output
$ python -m argilla.tasks.users.create_default --quiet

I have refactored a little bit some contexts.accounts functions related with passwords, maybe it could be applied later to some other PRs on the go.

[frascuchon]

Cool!