Fixes issue with permissiondenied error with a starting resource.

[arthurian]

@lduarte1991 I just looked at this branch and that's more or less what I had in mind. While I was testing it out, I discovered one unexpected thing with unpublished objects: rather than denying students access to unpublished to the object, it falls back to the admin hub view.

This PR changes that behavior so that if a PermissionDenied exception is raised when accessing an annotation target object, the exception is re-raised instead of defaulting to the admin hub view.

In addition to that change, I made these tweaks as well:

• Registered the LTIResourceLinkConfig model with django admin site.
• Modified the logging configuration so that the django SQL queries could be suppressed while debugging (change django.db.backends.level to ERROR rather than DEBUG).
• Added the ability to deselect a starting resource. For example, in Canvas we have a number of tools installed in the left-hand navigation and students have access to all annotation assignments. If an instructor mistakenly chooses a starting resource, we want them to be able to undo that.

[arthurian]

@lduarte1991 I just updated this PR with the ability to de-select a starting resource. Take a look and let me know if the changes look OK to you.

[lduarte1991]

looks great to me 👍 will merge in a bit.