Config for zxcvbn disabling

[magicznyleszek]

Proposed variable name: ENABLE_PASSWORD_ENTROPY_METER and the accompanying help text in the Django admin interface: Display an entropy meter and password quality suggestions whenever users change their passwords. This should be exposed to Front end through /environment endpoint and also be accessible in the templates (similarly to google_analytics_token or config.logo).

[noliveleger]

@magicznyleszek the wording in not the correct one in back end. I'll create a new PR to fix that and send to the front end what's suggested in the description of this issue.
cc @JacquelineMorrissette