Test and unpin Bandit

[sssoleileraaa]

Description

Bandit 1.6.1 seems to break our CI, see #428 for context. Once we figure out what's going on we will want to unpin from Bandit 1.5.1.

Note: Bandit 1.6.0 has a bug (see PyCQA/bandit#488) that 1.6.1 fixes but breaks our CI, which is why we pinned to 1.5.1

[emkll]

WIP branch here: https://github.com/freedomofpress/securedrop-client/compare/429-unpin-bandit

While testing, i've noticed (in both 1.5.1 and 1.6.1) that many files were being ignored. As part of closing this ticket, we should investigate and fix these issues, as the files being ignored contain a lot of important logic):

Run started:2019-06-18 15:53:03.292804

Test results:
	No issues identified.

Code scanned:
	Total lines of code: 3605
	Total lines skipped (#nosec): 0

Run metrics:
	Total issues (by severity):
		Undefined: 0
		Low: 0
		Medium: 0
		High: 0
	Total issues (by confidence):
		Undefined: 0
		Low: 0
		Medium: 0
		High: 0
Files skipped (16):
	./securedrop_client/api_jobs/base.py (syntax error while parsing AST from file)
	./securedrop_client/api_jobs/downloads.py (syntax error while parsing AST from file)
	./securedrop_client/api_jobs/uploads.py (syntax error while parsing AST from file)
	./securedrop_client/app.py (syntax error while parsing AST from file)
	./securedrop_client/config.py (syntax error while parsing AST from file)
	./securedrop_client/crypto.py (syntax error while parsing AST from file)
	./securedrop_client/db.py (syntax error while parsing AST from file)
	./securedrop_client/gui/__init__.py (syntax error while parsing AST from file)
	./securedrop_client/gui/main.py (syntax error while parsing AST from file)
	./securedrop_client/gui/widgets.py (syntax error while parsing AST from file)
	./securedrop_client/logic.py (syntax error while parsing AST from file)
	./securedrop_client/message_sync.py (syntax error while parsing AST from file)
	./securedrop_client/queue.py (syntax error while parsing AST from file)
	./securedrop_client/resources/__init__.py (syntax error while parsing AST from file)
	./securedrop_client/storage.py (syntax error while parsing AST from file)
	./securedrop_client/utils.py (syntax error while parsing AST from file)