Consider improvements to log verbosity, readability and parseability

[eloquence]

We often log events at multiple levels of the codebase, e.g., the queue, the database, the GUI. Interpreting logs can be challenging due to high verbosity and different wording, depending on where an event occurred. Log levels could perhaps also be used more intentionally.

In the 2020-10-22 tech meeting, we agreed that it would make sense to isolate some example events and discuss opportunities to optimize verbosity, readability, and parseability. The SecureDrop Client can serve as a first test case for arriving at a more consistent approach to logging across the SecureDrop Workstation.

[cfm]

Per #1552 (comment), it would be great to have consistent (ideally project-wide, not just component/repository-level) guidelines for log levels and perhaps formats.