Email Reminders and Dashboard Updates

[miknevinas]

This PR is pretty hefty. I'll try to summarize all the changes as neatly as possible:

1. The 'Sharing' view is overridden to do a couple of things:

• Send an email to a user (via a celery task) assigned as a 'Reviewer' on a particular page/item
• Add an optional 'Message' field that appends additional text to the email
• Display a warning if the user is operating in a directory with a lot of items, which will cause a short delay when granting permissions. The system recursion limit is also temporarily increased to accomplish this (Note: this functionality was taken from a separate PR which can be closed: Sharing permissions fix #584)

2. A system is added to send reminder emails to users about their assigned pages

• On initial assignment, the user is immediately sent an email via a celery task
• Users are sent a reminder email via a CRON job if they have not removed themselves as a reviewer within 5 days
• Both of these leverage the cache to track assigned items and their due dates, along with any custom messages
• Only 'Reviewer' roles are tracked throughout this process, though this can be changed if requested

3. Three new columns are added to the dashboard to display items that the logged-in user has modified, created or been assigned

• The 'modified' and 'assigned' columns require their own respective indexes to render, which are added in both the default and upgrade profiles
• All columns can now be hidden if desired
• The icons and verbiage have been removed as requested, though specifics on the new layout were never specified.

***NOTE:

The only way I could properly register the indexes was by rebuilding the catalog in Site Setup after running the upgrade. I tried both reindexing and re-cataloging programmatically in the upgrade profile, but neither of these worked. I'm open to ideas of how to fix this.