Switch version button may link to invalid url when we add/delete pages in latest version

Current switch button in right-up corner has bugs in the following two situations:

1. if we add a new versioned page which only exists in the latest version but not previous version, when we click the switch button, it goes to an invalid url.

e.g. we have page

http://samza.incubator.apache.org/learn/documentation/latest/new.html

the switch button in this page will link to

http://samza.incubator.apache.org/learn/documentation/0.7.0/new.html

which actually does not exist.

2. on the other hand, if we delete a versioned page in latest version, whenever we click the switch button in old page, it links to an invalid link too.

e.g. we have old page

http://samza.incubator.apache.org/learn/documentation/0.7.0/old.html

The switch button in this page actually links to

http://samza.incubator.apache.org/learn/documentation/latest/old.html

However, obviously, the link is invalid because we already delete this page.