Improvements to the Japanese unit pages

[Ben Bullock]

I've done the following improvements on the Japanese unit pages such as

https://www.sljfaq.org/cgi/junit_length.html.

* Trailing zeros are removed from decimals automatically

* Large numbers are no longer expressed using e+4 formats but using full numbers, with commas

* On inputting an invalid number such as "abc", a message for the user appears, rather than the previous "alert" system.

* It is now possible to input a zero in the forms.

Some work remains to be done to automate the upload but I have reached the point of exasperation with CSS, JavaScript, and friends for today so it will have to wait for a later date. Unfortunately it is difficult to automatically test JavaScript so I may have introduced new bugs.