[Master] Use sql database structure instead of schema.rb

* Use .sql format for database structure instead of schema.rb
* SQL format is super easy and fast to read and write. Creating a database structure from .sql file doesn't require rake to be loaded, thus it is faster at least on Travis where Zeus is not in use.
* Source: https://coderwall.com/p/x8exja, see 2) loading database without rake

What does not change:

* Almost everything
* Rake tasks, e.g. `rake db:migrate` works normally. It updates structure.sql instead of schema.rb

What does change:

* If you want to do schema dump, use `rake db:structure:dump` instead of `rake db:schema:dump`