Add black code formatter to all Python code in this repo

[eloquence]

We already enforce black via CI in the launcher directory, but not on the other Python code in this repo. Per our code formatting consensus we want to gradually implement use of black across all SecureDrop repos.

• with line length set to 100
• with an ignoreRevsFile to let us skip those revisions for purposes of attribution.

[eloquence]

(Suggesting we exclude isort here for now while we pilot its use in client, and potentially SDK/proxy repos.)

[eloquence]

Took a first stab at this in #569; the main immediate issues I see are:

• improving auto-discovery logic and ideally consolidating flake8/black runs
• a better workaround to flake8 error E231 after a successful black run psf/black#1289, if we want to preserve these trailing commas