Fix deletion of Post Model in Editor and Content screens

fixes #2999
- handle undefined argument in openModal function
- catch whether a model is deleted in Editor routes to aid transition
- move updateTags function to the PostModel
- add call to updateTags in delete-post modal
Merge pull request #3001 from darvelo/fix-delete-post

Fix deletion of Post Model in Editor and Content screens
working settings
themes computed property, select attributes, wire up upload modal
cleanup
add new files
cleanup