Switch Development Database via ENV var

This is useful if you use multiple development databases locally
and don't want to constantly `db:drop db:create` into
`discourse_development`.

Simply add `DISCOURSE_DEV_DB=whatever_db` as an ENV variable and
Discourse will use it in development mode.

config/database.yml

@@ -1,7 +1,7 @@
 development:
   prepared_statements: false
   adapter: postgresql
-  database: discourse_development
+  database: <%= ENV['DISCOURSE_DEV_DB'] || 'discourse_development' %>
   min_messages: warning
   pool: 5
   timeout: 5000