Add password strength indicator

[ebma]

Creates a new <PasswordStrengthTextField> component and uses it when creating a new account or changing the accounts password in account settings. A warning will be shown when choosing a password of strength 'Very Weak', 'Weak', or 'Fair' (it should be 'Good' or 'Strong')

• [x] Add zxcvbn dependency
• [x] Create <PasswordStrengthTextField> component
• [x] Use <PasswordStrengthTextField> when creating new account or changing password
• [x] Add <WeakPasswordConfirmation> dialog
• [x] Show <WeakPasswordConfirmation> dialog when choosing weak password

Closes #109.