Add password strength meter

[veeara282]

Closes #247

Add a password strength meter to the "New password" fields in ChangePasswordModal and ResetPasswordModal using a PasswordStrengthModalInput, which composes ModalInput. This component will always display validation text, even when the password is valid.

Password strength indicator colors:

• Weak/invalid: #f05452 (tangerine)
• Okay: #43c4d9 (aqua)
• Good: #388ec5
• Great: #2c58b1 (bluejay)

TODO:

• Allow {Reset,Change}PasswordModal to read the validation result (true/false) of the PasswordStrengthModalInput
• Color shouldn't be red when the password is valid

[veeara282]

We should also consider adding some default and personalized strings to the user_inputs argument.

The optional user_inputs argument is an array of strings that zxcvbn will treat as an extra dictionary. This can be whatever list of strings you like, but is meant for user inputs from other fields of the form, like name and email. That way a password that includes a user's personal information can be heavily penalized. This list is also good for site-specific vocabulary — Acme Brick Co. might want to include ['acme', 'brick', 'acmebrick', etc].

[ibeckermayer]

Ignore, I'm just clearing out old review requests in my GitHub account.