Set drop-down selected values using select2's API.

If we only manipulated the DOM, the selected state wouldn't be reflected
in the select2 widget.
Moved decorators shared across apps to pootle.core.decorators.
Use explicit relative imports.
Moved decorators to its own module.
Organized imports.
Remove quality checks' drop-down when performing searches.
Require `filter=checks` when filtering checks.

Although this doesn't apparently add anything exciting, it'll help to keep
the editor's filtering query strings aligned across different types of
pages.
Fixed background styling issue.
Avoid displaying repeated results.