config: auto generate secret key if none given.

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

config/docker/docker-compose.test.yaml

@@ -16,7 +16,6 @@ services:
       - DATABASE_URL=postgresql://danbooru:danbooru@postgres/danbooru
       - ARCHIVE_DATABASE_URL=postgresql://danbooru:danbooru@postgres/danbooru
       - PARALLEL_WORKERS=8 # number of parallel tests to run
-      - DANBOORU_SECRET_KEY_BASE=1234
       - DANBOORU_AWS_SQS_ENABLED
       - DANBOORU_TWITTER_API_KEY
       - DANBOORU_TWITTER_API_SECRET