[frontend] Discarding finished tasks from the store

    - They would cause memory leak if user keeps
      pounding on preview/render links
[frontend] Simplifying task polling logic
[frontend] Fixed navbar detacched when scrolling main port
[frontend] Adding a render route for templates

    - This organizes the code better, since handling
      the render action was before on the templates
      controller and now belong to the render route
[frontend] Added template details