Reset slug input text box after invalid input.

No Issue
- When an invalid, all whitespace slug is entered into the
  slug input in the post settings menu, it's rejected but
  the input's value still remains the same.  This resets the
  input back to its original value.
- Added test for the above behavior.
- Only show success notification if slug was actually changed.
- Convert whitespace from tabs to spaces in post-settings-menu.hbs
Merge pull request #3767 from jaswilli/clear-ws-slug