Automatically generate a random secret key for `Danbooru.config.secret_key_base`
if no key is specified.
This so that you can run Danbooru in a Docker container with zero
configuration.
This removes support for the ~/.danbooru/secret_token file and the
SECRET_TOKEN environment variable. If you used either one of these, you
must copy the value either to DANBOORU_SECRET_KEY_BASE in .env.local, or to
`secret_key_base` in config/danbooru_local_config.rb.
# .env.local
DANBOORU_SECRET_KEY_BASE=<value>
# config/danbooru_local_config.rb
def secret_key_base
# <value>
end
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