[imp] Allow server-relative URLs in redirection. Fixes #4255

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

line 75: tabs removed

check redirect found whether published
Add trigger to com_content category model