Move issue board models to boards dir

What does this MR do?

Fixes gitlab-org/gitlab-ce#61293.

Moves classes in app/assets/javascripts/vue_shared/models to app/assets/javascripts/boards/models, since they are mostly used only by issue boards.

One component, ee/app/assets/javascipts/epic/.../sidebar_labels.vue uses labels.js, so labels.js was duplicated into ee/app/assets/javascrips/vue_shared/models/label.js, to avoid changing the epics codebase and to retain compatibility with CE backports.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content