make SECRET_KEY_BASE mandatory

[DavideCarvalho]

I had to move Docuseal from a on prem environment to a hosted one on AWS that can autoscale

We started having problems when the project started autoscaling because each instance would have their own encryption hash auto generated

I think it's better to make it explicitally mandatory instead of optional - and therefore, generating your own hash if it's not provided - it's better to make it clear that you DO need it, otherwise if you try to migrate your application to another place your screwed