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.
The value field of ThemeField is only used when viewing a diff in the staff action logs and local theme editing. value is being serialized into the theme index as well, which is not used. It's a huge amount of JSON that we can cut by removing it.
This also breaks up the various theme serializers into separate classes so they autoload properly (or at least restart the server on edit)
