Add password strength meter for password protected content

[paulpleasant]

I think it could be really helpful if the Page Passwords (whenever you want to have a page be password-protected) had a built-in password complexity checker. It's easy for these password-protected pages to have weak security with potentially sensitive information behind them - and I think it would help everyone if it made users create or generate a more complex password for pages (similar to WordPress local user passwords generated).

[desrosj]

This is actually a really neat idea.

We already have password strength meters everywhere else a password can be set/changed, so I think adding one here would bring some consistency.

I don't know that a minimum password strength should be enforced by default here, but maybe someone could enable that with a filter if desired.

This will likely need to be explored within the context of Gutenberg in the GitHub repository, but going to leave this open here for now.