Add schema.js

closes #1398
closes #1399
closes #1400

- added schema.js with database version '000'
- refactored migration to use schema.js
- if new table is added to schema.js and databaseVersion is increased, table will be added
- if new table is deleted to schema.js and databaseVersion is increased, table will be deleted
- alter table from issue #1400 is delayed until knex supports column modification
- changed import pre checks to work again (will be refactored separately)
- added basic PostgreSQL support (Attention: not supported/tested)