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

INSTALL.debian

@@ -100,13 +100,6 @@ sudo -u danbooru git clone git://github.com/sstephenson/ruby-build.git ~danbooru
 sudo -u danbooru bash -l -c "RUBY_CONFIGURE_OPTS=--disable-install-doc rbenv install --verbose $RUBY_VERSION"
 sudo -u danbooru bash -l -c "rbenv global $RUBY_VERSION"
 
-# Generate secret token and secret key
-echo "* Generating secret keys..."
-sudo -u danbooru mkdir ~danbooru/.danbooru/
-sudo -u danbooru sh -c 'openssl rand -hex 32 > ~danbooru/.danbooru/secret_token'
-sudo -u danbooru sh -c 'openssl rand -hex 32 > ~danbooru/.danbooru/session_secret_key'
-chmod 600 ~danbooru/.danbooru/*
-
 # Install gems
 echo "* Installing gems..."
 sudo -u danbooru bash -l -c 'gem install --no-ri --no-rdoc bundler'