vpnmain.cgi: Fix for bug13029 - add base64 encoding to IPSec cgi page

vpnmain.cgi: Fix for bug13029 - add base64 encoding to IPSec cgi page

- This adds the base64 encoded PSK into the config file and when the ipsec.secrets file
   is created the PSK is base64 decoded to write it to the file. The ipsec.secrets file
   surrounds the PSK with single quotation marks so that character is not allowed to be
   used in the PSK but anything else can be.
- Tested out on my vm system and shown to be working. New PSK with various characters
   characters including commas was base64 encoded before putting into the config file
   and therefore was accepted by the code. If a single quotation mark was used in the
   PSK then the error message about invalid characters was shown.

Fixes: Bug13029
Tested-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>