Fix SUBSTITUTION_URL value

[AlexGacon]

Description

My first implementation, which was a copy/paste of the NGINX_BASE_URL value with an additional "/", is not working since the GeoServer oauth2 configuration does not match the one expected by GeoNode

Type of Change

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• [x] Bug fix
• [ ] New feature
• [ ] Documentation update
• [ ] Refactoring
• [ ] Other (please describe)

Related Issue

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closes #132

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• The feature or bug fix has been discussed and documented in an issue beforehand.

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[AlexGacon]

@mwallschlaeger if no additional PR is expected, it would be great to have a release as soon as possible.