Allow reinstalling an extension from a remote repository in the extension manager

This should be helpful in different use case:

• reinstall an upgraded snapshot of the same extension while testing
• repair a broken extension that have been locally corrupted

The workaround is to use http://extensions.xwiki.org/xwiki/bin/view/Extension/Extension+Tweak and do the following:

• uninstall
• clean the local extension using Extension Tweak
• install