Enforce linting standards in CI

[conorsch]

The repository contains files of several common types, including:

• [ ] YAML (*.sls and *.top Salt files)
• [ ] salt-lint (see https://github.com/warpnet/salt-lint)
• [x] Python scripts (see https://github.com/freedomofpress/securedrop-workstation/pull/62)
• [x] Bash scripts

Let's enforce linting across all those files using the appropriate tool, e.g. yamllint, flake8, and shellcheck, respectively. Linting the Salt files may require some creativity, given the Jinja declarations. See here for some bright ideas in that direction: https://stackoverflow.com/questions/31234541/linting-salt-states-without-running-them

[conorsch]

Examples for each linting strategy can be found in the SecureDrop repository:

• yamllint https://github.com/freedomofpress/securedrop/blob/58ab66e76f6a5fb23d8bf3d9c969f4e4d89d37d9/Makefile#L64-L70
• shellcheck https://github.com/freedomofpress/securedrop/blob/58ab66e76f6a5fb23d8bf3d9c969f4e4d89d37d9/Makefile#L72-L86
• flake8 https://github.com/freedomofpress/securedrop/blob/58ab66e76f6a5fb23d8bf3d9c969f4e4d89d37d9/Makefile#L45-L51

[rocodes]

backlog pruning session:

• [x] shellcheck is now enforced
• [ ] adding at least yaml lint would be good, probably also salt linting (need to investigate)