Include self in Course#tabs_available cache_key
Include self in Course#tabs_available cache_key

This adds the current object into the array used for generating
the cache key for #tabs_available. When the Course is updated
these tab caches will be invalidated giving users the most accurate
access to tabs.

I wasn't able to recreate this locally with the way caching was set
up but it is present in production so testing it may be difficult.

fixes LS-1773
flag=none

test plan:
- Create a course with a student
- Publish the course
- Open the course as a student in another window
- Verify the Announcements tab is not visible
- Create an announcement in the course and save it
- Refresh the student's view
- See the Announcments tab is visible to the user

Change-Id: I03049db4445d1650fbf20d9ac9b598631cfc9ca5
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/258384
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Robin Kuss <0b206b54367b2de0334896ea45826f0e3194bd35@instructure.com>
QA-Review: Robin Kuss <0b206b54367b2de0334896ea45826f0e3194bd35@instructure.com>
Product-Review: Eric Saupe <e7287cf05ae62bc45889f17a9d34b8fae8d5afd1@instructure.com>

instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms