[imp] Allow server-relative URLs in redirection. Fixes #4250

Suppose you have a web server using Joomla where this web server is both reachable via http://www.example.org/ and http:/example.org/. With the current implementation of the redirection plugin you need to define two redirections within the redirect manager component for every page you want to automatically redirect from its old location to its new location:

old page: http://www.example.org/path/to/old/page.html => new page: http://www.example.org/path/to/new/page.html
old page: http://example.org/path/to/old/page.html => new page: http://example.org/path/to/new/page.html

With my proposed file change it would be sufficient to define only one redirection in this case:

old server-relative page: /path/to/old/page.html => new server-relative page: /path/to/new/page.html

concatenation spacing and { in a new line

changes made due to reply from zero-24

check redirect link found whether published

After searching for a redirect for the complete URL check not only whether there was a match at all but check also whether this redirect ist published. If either is not the case try to find a redirect with the server-relative URL.