embed background image in redirect template

embed background image in redirect template

Embed the IPFire background image into the redirect template
directly via CSS instead of loading it from somewhere else.
This is necessary because of Content Security Policy (CSP).

This patch inserts the base64 encoded image during build so
nothing needs to be updated twice in case background image
changes.

It supersedes first to fourth version of this patch and has
been successfully tested during a clean build.

Fixes #11650

Signed-off-by: Peter Müller <peter.mueller@link38.eu>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>