Fix 403 Error Page Aesthetics and Ensure Consistent URL Generation

[JigyasuRajput]

Cambios propuestos:

• Fixed the 403 error page to match the site's aesthetics.
• Ensured consistent URL generation and validation in the code.

Temas tratados:

• Closes 403 cuando quiero agregar una empresa ya estando asociado a otra #567

Pruebas realizadas:

• Ran the tests successfully with no errors.
• Tested on Ubuntu 20.04

Cómo probar los cambios:

1. Join a company on the site.
2. Manually visit the URL https://www.python.org.ar/empresas/agregar/ to confirm the 403 page is displayed with the updated aesthetics.

Gracias por contribuir con el proyecto.

Estamos pendiente de revisar los cambios propuestos.

[facundobatista]

It looks there are changes here that are also in other proposed PR?

Also please double check changes, there are some that are not really changing anything useful, thanks!