Add pluggable function to enforce password strength

[pishmishy]

Patch adds an extra option (disabled by default) to force users to chose secure passwords. A plugin replaceable function is called at the appropriate time to check the strength of the password. I realize that this functionality could be achieved through a plugin but I'd like to make the case that the extra security gained against brute force attacks makes it worth including. See also #4470 (Ideally I should really rewrite the function to replicate that javascript function).

[pishmishy]

Ignore my comment on #4470. With after thought I don't think that Javascript function isn't great. checkRepetition() isn't well documented, the algorithm isn't clear (why check for symbols, numbers and then again for symbols and numbers - surely the score would take that into account naturally?).

Preferring my patch for now.

[darkdragon]

Well, since you are doing this, then ignore my other comment.

[JDTrower]

A patch has been posted on #4470 that includes the functionality requested in this ticket. That patch needs testing.

[pishmishy]

Thanks. Closing as duplicate of 4470.