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)

To clarify the usage of `./dspace database migrate [version]`, it can be used to test DB version identification as follows:

1. Clean existing DB: `./dspace database clean`
2. Create a specific version of a new DB: `./dspace database migrate 1.4`
3. Remove the Flyway table: `DROP TABLE schema_version;`
    * For Oracle: `DROP TABLE "schema_version";`
4. Test if we can identify the version of the DB correctly: `./dspace database info`
    * Should say "Your database looks to be compatible with DSpace version ___"
I did PR in a big hurry.