Implement Migration Paths for Flag 6.x

Problem/Motivation

Flag 8.x does not currently supply a migration path from earlier versions.

Proposed resolution

Instead of relying on hook_update_N() to migrate configurations from previous versions, the new Migrate API should be used. This will have the additional benefit of allowing a direct migration from Flag 6.x to 8.x.

Remaining tasks

Create patch.

User interface changes

None.

API changes

None.

Data model changes

None.