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.

config/database.yml

@@ -0,0 +1,39 @@
+# Don't edit this file. To override this file, set `DATABASE_URL` in .env.local
+# instead. Example:
+#
+#   DATABASE_URL=postgresql://danbooru:password@localhost/danbooru2
+#
+# https://guides.rubyonrails.org/configuring.html#configuring-a-database
+
+default: &default
+  adapter: postgresql
+  username: danbooru
+  host: localhost
+  url: <%= ENV["DATABASE_URL"] %>
+
+production:
+  <<: *default
+  database: danbooru2
+
+development:
+  <<: *default
+  database: danbooru2
+
+test:
+  <<: *default
+  database: danbooru2_test
+
+archive_production:
+  <<: *default
+  database: archive_production
+  url: <%= ENV["ARCHIVE_DATABASE_URL"] %>
+
+archive_development:
+  <<: *default
+  database: archive_development
+  url: <%= ENV["ARCHIVE_DATABASE_URL"] %>
+
+archive_test:
+  <<: *default
+  database: archive_test
+  url: <%= ENV["ARCHIVE_DATABASE_URL"] %>