Move issue board models to boards directory

What does this MR do?

Fixes gitlab-org/gitlab-ce#61293

1. Moves two classes in javascripts/vue_shared/models to javascripts/boards/models, because they are primarily used with boards.
2. Duplicates ...vue_shared/models/label.js to ee/.../javascripts/vue_shared/models, because one class in epics still depends on it. This will eventually be refactored while we move to Vue.
3. Updates dependencies and tests to reflect the new paths.

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