Make inline "pylint: disable=" notes more clear

Switch ambiguous statements such as "pylint: disable=W0621" with clearer disable statements such as "pylint: disable=redefined-outer-name". I propose that we should be using the clearer form throughout the codebase, so code reviewers know what's going on and coders don't blindly carry over disable statements that no longer apply.

This is also the recommendation made here: https://github.com/edx/edx-platform/blob/master/docs/en_us/developers/source/testing/code-quality.rst
I don't think we need to disable `fixme` errors anymore, ever since https://github.com/edx/edx-platform/pull/5814 was merged.