Flyway fixes for Oracle (DS-2244, DS-2250, DS-2251)

* Refactor Flyway DatabaseUtils to properly search by Oracle "Schema" (i.e. User), see getSchemaName()
* Also add a (hidden) ability to initialize an *older* version of a DSpace DB for testing purposes (`./dspace database migrate [version]`, e.g. `./dspace database migrate 1.7`)

I believe this may fix most, if not all of these tickets:
* https://jira.duraspace.org/browse/DS-2244
* https://jira.duraspace.org/browse/DS-2250
* https://jira.duraspace.org/browse/DS-2251

This also replicates the tiny change in PR#727 for easier testing.

Needs more testing on both Oracle & PostgreSQL (to ensure the refactoring doesn't hinder PostgreSQL in any way)