EPIC: Give a thread its own URL and page

[tschaffter]

Background: The visualization of a thread currently happens in the right sidenav.

There are a few issues with this approach alone:

• The thread creation form spans on an entire page, but the thread visualization is in the sidenav, which is less wide than a page (different format).
• The sidenav enables the visualization of a thread while continuing to navigate. However it is more comfortable for the user to review the thread on a page (more can be seen without scrolling/user action).
• There is currently no way to get a URL that targets a given thread, which would be useful for sharing to other users
  • Enable to copy thread URL without opening the thread #448
  • Enable to copy message URL #449