Move issue board models to boards directory

This is part of refactoring issue boards to Vuex (&1278 (closed)).

The classes in app/assets/javascripts/vue_shared/models are only used in issue boards:

$ grep -lr 'vue_shared/models' app/assets/javascripts
app/assets/javascripts/boards/index.js
app/assets/javascripts/boards/models/list.js
app/assets/javascripts/boards/models/issue.js

Therefore we should move them to app/assets/javascripts/boards/models/.