Allow customization for db migration

[ZakarFin]

Moved fi.nls.oskari.servlet.WebappHelper to new path org.oskari.init.OskariInitializer. This file is usually not customized so the change shouldn't have any effect on most instances. However we do have an instance that had WebappHelper copied to the application codebase with minor change to run migrations a bit different than in the built-in one. Mainly using other db-credentials for migrations than the app usually uses at runtime.

This adds a way of doing just that without the need to copy the whole file and maintain after changes. With this the application can add a custom class that implements org.oskari.init.CustomMigration and define oskari-ext.properties with:

db.flyway.migrationCls=foo.bar.CustomMigrationImpl

On launch the application would then run foo.bar.CustomMigrationImpl.migrateDB() instead of the normal migration process.