Add translation context on password strength strings

Hi, currently the password strength module uses 'strong' as string to indicate the best password strength. That's good, but in this specific context the word strong in italian language has a different translation compared to the html "strong" element. So we need to differentiate it using a translation context.

So now I've patched user.module with a custom context.

N.B: In Italian language, the word "strong", when used to indicate the bold style, is translated with "grassetto". Instead, when the word "strong" is used to specify the strength of something, is translated with "forte".