Also fixes a few untreated exceptions and remove unused variables.

Signed-off-by: Giovanni Cimolin da Silva <giovannicimolin@gmail.com>

This commit adds support for the LTI 1.3 Access token endpoint, as detailed in the IMS Security Framework.
The token is generated using the consumer's private key (stored in the XBlock) after validating the message sent by the LTI Tool using it's public key.

Signed-off-by: Giovanni Cimolin da Silva <giovannicimolin@gmail.com>

Features:
Implement LTI 1.3 launch support.
Add LTI 1.3 passthrough views as LMS plugins.

asadiqbal08: Updated branch as per suggestions
asadiqbal08: Addressed PR feedback
mattdrayer: Rebased branch, addressed merge conflict