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
Nitpick: I believe that it's possible to disable multiple pragmas on one line, so that this could be:
```python
# pylint: disable=missing-docstring, redefined-outer-name
```
Is that something we want to do in this pull request? (Or at all?) Personally, I think it's cleaner, but this may not be the time or the place for this change.