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.

lib/capistrano/tasks/symlink.rake

@@ -3,7 +3,6 @@ namespace :symlink do
   task :local_files do
     on roles(:app, :worker) do
       execute :ln, "-s", "#{deploy_to}/shared/config/danbooru_local_config.rb", "#{release_path}/config/danbooru_local_config.rb"
-      execute :ln, "-s", "#{deploy_to}/shared/config/database.yml", "#{release_path}/config/database.yml"
       if test("[ -f #{deploy_to}/shared/config/newrelic.yml ]")
         execute :ln, "-s", "#{deploy_to}/shared/config/newrelic.yml", "#{release_path}/config/newrelic.yml"
       end