Move issue board models to boards directory
What does this MR do?
Fixes gitlab-org/gitlab-ce#61293
- Moves two classes in
javascripts/vue_shared/models
tojavascripts/boards/models
, because they are primarily used with boards. - Duplicates
...vue_shared/models/label.js
toee/.../javascripts/vue_shared/models
, because one class in epics still depends on it. This will eventually be refactored while we move to Vue. - Updates dependencies and tests to reflect the new paths.