Cleanup syntax and linting issues for backend code

[mbeacom]

Description

The goal of this PR is to cleanup some of the backend code to adhere to flake8/pylint standards.
Additionally, this PR removes many unused files and imports.

Checklist

• linter status: 100% pass
• changes don't break existing behavior
• commit message follows commit guidelines

[owocki]

seems okay to me if it's just linting.. this is gonna cause a lot of merge conflicts.

two questions:

1. how did you test this, if at all?
2. is can you update the contrib guidelines with a list of commands i can run to lint my PRs before i submit them?
3. any updates to travis required?

[codecov]

Codecov Report

Merging #577 into master will increase coverage by 0.05%.
The diff coverage is 11.11%.

@@            Coverage Diff             @@
##           master     #577      +/-   ##
==========================================
+ Coverage   29.05%   29.11%   +0.05%     
==========================================
  Files          93       88       -5     
  Lines        4890     4880      -10     
  Branches      571      571              
==========================================
  Hits         1421     1421              
+ Misses       3422     3412      -10     
  Partials       47       47

Impacted Files	Coverage Δ
app/credits/apps.py	0% <ø> (ø)	⬆️
app/marketing/management/commands/sync_slack.py	0% <ø> (ø)	⬆️
app/app/settings.py	88% <ø> (-0.47%)	⬇️
app/marketing/management/commands/pull_github.py	0% <ø> (ø)	⬆️
...arketing/management/commands/ingest_slack_users.py	0% <ø> (ø)	⬆️
app/legacy/urls.py	100% <ø> (ø)	⬆️
app/gas/admin.py	100% <ø> (ø)	⬆️
app/linkshortener/admin.py	100% <ø> (ø)	⬆️
app/marketing/apps.py	0% <ø> (ø)	⬆️
app/retail/helpers.py	28.57% <ø> (+11.9%)	⬆️
... and 38 more

---

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[mbeacom]

@owocki Agreed, regarding merge conflicts... but we need to get the BE code to a place we can automatically check linting at some point without introducing a stoppage point for every change initiated against the files that were modified here.

This is the first step in one of multiple to get the BE to a point we can lint and not hinder users from contributing without resolving previously existing linting errors.

There are no needed changes to Travis yet, but I can add documentation to the contributing doc regarding linters to run, i.e.: flake8, pydocstyle, pylint, pycodestyle, etc.
I recommend using sublime/atom/vscode plugins to actively monitor these types of errors/warnings while making changes, but most of these can be ran from terminal (but may result in quite a few errors at the moment).

As for testing, I tested this locally and all seemed fine. Most of the changes aren't modifications to logic, but rather some syntax changes and removal of unused imports and skeleton files.

The only thing that should be outstanding following this PR for CI enforced linting should be the functions that violate mccabe checks for complexity, number of local vars, and number of method arguments (from an error standpoint).

[owocki]

lgtm

[owocki]

There are no needed changes to Travis yet, but I can add documentation to the contributing doc regarding linters to run, i.e.: flake8, pydocstyle, pylint, pycodestyle, etc.
I recommend using sublime/atom/vscode plugins to actively monitor these types of errors/warnings while making changes, but most of these can be ran from terminal (but may result in quite a few errors at the moment).

+1