Linting and initial unit tests

[glennmatthews]

These diffs have grown a bit large, so I figured I should open a PR now rather than later.

• Basic unit test framework, including simple tests for the REST API (more tests to be added in a future PR to finish addressing #1)
• Enable pylint and fix or disable all extant warnings
• Enable pydocstyle and fix or disable all extant warnings
• Enable black, bandit and all of the above as part of invoke tests (fixes #2)
• Travis-CI build now includes invoke tests.
• Work around django-rq version issue impacting NetBox by explicitly installing django-rq==2.3.2 for now.
• Make REST API consistently use slugs for object references, instead of mix of slug and name references (fixes #12).
• Disallow PUT and PATCH requests to the REST API - onboarding tasks can only be created, read, and deleted, not updated via the API.