Permalink in a ToDo open in a new browser tab

[DHaussermann]

When a user clicks a permalink that is in the contents of a ToDo item, the link opens in a new browser tab. The expected behaviour is that the user should navigate to the post permalink view in the existing tab and the ToDo RHS should remain open.

This is working as expected in desktop app. Only browser seems affected.

1. Make a post from a browser
2. Click the actions post menu option and select Add ToDo
3. Add some text below the permalink and save
4. Open the ToDo RHS
5. Click the PermaLink in the ToDo
   Observed: Post permalink view is shown in new tab.

After some isolation testing using Mattermost 6.5.0 and various versions of To Do - this seems to be a regression for v0.5.1 --> v0.6.0 as the issue starts occurring without any change to the server/webapp code.
This also happens regardless of weather the App Bar is enabled or not.