Add method to clear mako template lookups

Quick and dirty fix (edxapp.startup.run is called twice, once before
loading the configuration, so the paths defined in lms.startup are
basically ignored).
Do not allow students to pin/unpin forum threads

The wrong permission was being checked for pinning/unpinning, so a
sufficiently clever student could pin and unpin threads.

JIRA: FOR-499
Ensure account creation succeeds before creating comments service user

Also improves handling in cases where account creation did succeed, but
comments service user creation did not.

Depends on commit 88abdd8a0b20bc9816f487b25abdea1953f5661d in https://github.com/edx/cs_comments_service

JIRA: FOR-522
Merge pull request #2960 from edx/gprice/FOR-499

Do not allow students to pin/unpin forum threads
Merge pull request #2957 from edx/jsa/forum-user-sync

Ensure account creation succeeds before creating comments service user
Merge pull request #2840 from ionis-education-group/double-edxmako-startup

Add method to clear mako template lookups
mentoring-noworkbench: Remove import of workbench module (moved to SDK)