Added functional tests for editor: tag editor, image uploads, post settings

references #2273
- test tag creation and tag deletion
- tests image uploader appears after typing `![]()` in editor
- tests image URL matches url inside `![](url)`
- tests all input elements of post settings menu
Merge pull request #2395 from mattse/mattse#2273

Added functional tests for editor: tag editor, image uploads, post settings