refactor server_variables
1. move functions to separated files
2. render HTML at once
3. refactor function name to PMA_getHtmlForSubPageHeader
Merge branch 'master' of https://github.com/phpmyadmin/phpmyadmin into server_refactor2
remove server_variables_doc.php and add a function to return doc links
Merge pull request #562 from xmujay/server_refactor2

refactor server_variables