config: set default database connection settings.

Set sensible defaults for connecting to the database. By default, we try
to connect to the `danbooru2` database running on localhost as the
`danbooru` user. These are the defaults recommended by the install
guide.

If you need to change the database settings, set DATABASE_URL in
.env.local or on the command line:

   # .env.local
   DATABASE_URL=postgresql://danbooru:password@localhost/danbooru2

   # command line
   $ DATABASE_URL=postgresql://danbooru:password@localhost/danbooru2 bin/rails server

This eliminates the need to copy script/install/database.yml.templ to
config/database.yml during installation and during deployment. This is
so that Danbooru works out of the box without extra configuration. In
particular, this is so that we can run Danbooru in a Docker container
without having to set DATABASE_URL.

INSTALL.debian

@@ -139,7 +139,6 @@ mkdir -p /var/www/danbooru2/shared/data
 mkdir -p /var/www/danbooru2/shared/data/preview
 mkdir -p /var/www/danbooru2/shared/data/sample
 chown -R danbooru:danbooru /var/www/danbooru2
-curl -L -s $GITHUB_INSTALL_SCRIPTS/database.yml.templ -o /var/www/danbooru2/shared/config/database.yml
 curl -L -s $GITHUB_INSTALL_SCRIPTS/danbooru_local_config.rb.templ -o /var/www/danbooru2/shared/config/danbooru_local_config.rb
 
 echo "* Almost done! You are now ready to deploy Danbooru onto this server."