New Super User setting to edit the Piwik hostname #3230

We should

• save the host name + path during the initial installation,
• use this value in password reset link, archive.php archiving script, etc.
• have a new setting in General Settings to edit the Piwik URL.
• when the Host is different from recorded PiwiK URL, display yellow warning with link to edit the hostname (to make migration still easy for users)

See also related #3220

see #3080