Allow sqlite migrations for non-default databases.

[rylev]

Fixes #2607

The database will be selected by the name of the migration file. This makes the name of the migration file significant, whereas previously it was not. If the name of the file and a known database don't line up, spin up will fail with an error message.

[rylev]

@itowlson thanks for raising these concerns. I personally don't have strong feeling about what the right UX is here. I'd just want to ensure that it's easy to tell when the user is passing a file vs when they're passing raw SQL (since we support both). Right now the @ is used to disambiguate so perhaps we should keep the leading @. Perhaps a : between file name and databasename (i.e., @file.sql:database-name)?

[itowlson]

@file.sql:database-name works for me. Thanks!

[rylev]

@itowlson ok done!