Use the IssueFinder to filter by labels in the API

Issues are filtered by labels by a custom filter method filter_issues_labels which matches if any of the labels matches. However, the IssueFinder already provides a functionality to filter by labels but only matches if all labels matches.

Proposed Change

Use the IssueFinder label filtering method in the issues to avoid duplication. The /groups/:id/issues already uses the IssueFinder therefore. Changing this behavior would also unify the API behavior.

/cc @smcgivern