Why this change?
Importing theme with the `bundle` params is used mainly by
`discourse_theme` CLI in the development environment. However, we do not
want migrations to automatically run in the development environment
and instead want the developer to be intentional about running theme
migrations. As such, this commit adds support for a
`skip_migrations` param when importing a theme with the `bundle` params.
This commit also adds a `migrated` attribute for migrations theme fields
to indicate whether a migrations theme field has been migrated or not.
