mv celery_tasks session info from contextproc to tasks

- The function for getting session info for celery tasks was
  in webui.context_processors. This was moved to webui.tasks.
- Added task status and entity URL to task info from session.
  This info comes from the Celery API.
celery tasks status display page

Page for displaying status of celery tasks, with links to the
entities and files and stuff.
Tasks are in session, and they currently don't expire automatically
so included "dismiss" links.
display number of various statuses in base template

Display the various status with counts of how many items exist
for each status (SUCCESS, PENDING, FAILURE), with link to tasks
display page.
pass request object into template context