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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.

updated bleach

Added img to bleach safe tags

Added img to bleach safe tags

Added img to bleach safe tags

added test

added test

added test

added test

added test for attr image

added test for attr image

fixed style bug

Updated format xoe

removed extra formatting

Updated to run tests on both python 2 and 3.

PROD-774

Overview
---------
This pull request makes this XBlock extensible for
adding parameters to the LTI provider in addition
to the existing ones.

Why
---
A customer needs `team` and `cohort` informaiton
to be passed through the LTI channel to simplify
integration with their LTI provider. At the same
time we'd rather not fork the XBlock, so an
Open-Closed model is needed.

More Detailed Documentation
---------------------------
I've added a couple of sections to `README.rst`
to document specifying design and
configuration steps of the new LTI parameter
processors. Please check it out for more
information about the changes that this pull
request introduces.

Example Usage
-------------
I've written a package that we'll need to use for
our customer to pass some personal user
information in addition to the cohort and team
info: https://github.com/appsembler/tahoe-lti/pull/1

Omar Al-Ithawi @ Appsembler

Add context_label and context_title to LTI POST parameters; Add required requirements; add some docker devstack instructions;

This allows this xblock being hooked into get_course_lti_endpoints for
discovering LTI result/outcome endpoints.

Updating code to look at UserProfile.language_proficiencies; updating tests; iterating version to 1.0.8

Use django.utils.timezone.now() when determining if the due date of a component has passed to avoid tz-aware/naive datetime comparison bug