Fix content preview actions and add tests

Closes #2798
-fetch full model for active post in Backbone content preview view
-remove unnecessary type check in Models.Post.saving
-add functional tests for all post settings menu actions in editor screen
-add functional tests for all post actions in content preview
 screen for posts with and without tags
Merge pull request #2799 from jaswilli/issue-2798

Fix content preview actions and add tests