Implement AiMigration core service (skeleton + title)

MR is created. Basic skeleton of the migrater service is there. I had AI write some Kernel Tests. Tests of the `convert()` method got too complicated and time-consuming to correct/update, so I pulled them for now.

... and I just realized that technically it should be "migrator" and not "migrater." Doh.

[Removing LINCS as customer from my attributions.]

Thank you @majorrobot. Skeleton looks good and straightforward right now. Tests are passing. I left a comment about the convert function test.

Thanks for your review @dmundra. I've merged it in. Onward and upward!