[FEATURE] Installer: Remove (unknown) form target URL

The domain on which Jirafeau is running, is not yet
defined while installing the project. In order to build valid
action links for the form, the script generated a possible URL
using the request header. This method may fail for proxies
or while using HTTPS or due to spoofing or…

In HTML5 we can safely omnit the action attribute (see
http://stackoverflow.com/a/9678030), which will cause the browser
to send the request to the exact same URL again.

This way we can avoid the URL guessing during the installation.

Refs #79