To correct the, in our case, single database entry that snuck in without
a UUID during the schema change.

Steps:
- Make the field editable
- Add `config_id` to all models missing it
- Make the field noneditable again

This implements the endpoint that LTI tools use to return the deep linking configuration to the LMS, along with the `ltiResourceLink` type to allow testing with demo tool.

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

A previous PR (#130) added a new unique field with a default value to the
LtiConfiguration model. Such cases need special handling in migraiton, however
the previous PR did not include that. This commit fixes the migration to include
the special handling for new unique fields.

Remove the OIDC view from XBlock and return OIDC url directly.

* bump up the version

* update changelog

As stated by @giovannicimolin:

  Due to a recent change in `Chrome`_ and `Firefox`_
  and the way they handle cookies, it’s not possible
  to access cookies marked as ``SameSite=None`` if
  they are not also Secure.

  This is to help avoid a few CSRF attacks.

  Secure cookies are only available when the request
  is done through `HTTPS`_, which is not the case on
  the devstack.

The workaround for this behaviour is to set
``DCS_SESSION_COOKIE_SAMESITE`` from ``'Lax'``
to ``None`` in the studio container's ``devstack.py``,
when using tools such as ``ngrok``.

This commit adds a note about this to the ``README.rst``
of the repository.

.. _Chrome:: https://twitter.com/ChromiumDev/status/1293236234932846596
.. _Firefox:: https://hacks.mozilla.org/2020/08/changes-to-samesite-cookie-behavior/
.. See ``Secure`` Attribute as a reference
.. _HTTPS:: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Set-Cookie

Also fixes a few untreated exceptions and remove unused variables.

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

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;

updated readme

fixed link

better fix for link