Find and migrate to maintained alternative to cssmin

[redshiftzero]

Description

We use cssmin to minify CSS, but it is no longer maintained: https://github.com/zacharyvoase/cssmin (last commit 4 years ago).

csscompressor is a similar project that is maintained, we might consider using that instead: https://github.com/sprymix/csscompressor.

User Stories

As a SecureDrop maintainer, I want my dependencies to be well-maintained so that breakage or security vulnerabilities are noticed and promptly fixed.

[trankmichael]

I looked for references of cssmin but found that last reference to it was removed from the app code in 2ad8678#diff-88383f21e19c9be2cf4d9b6a81184f7a . When I uninstalled it, removed it from requirements, and reprovisioned , all the tests passed.

While looking at a replacement for minifier, I was thinking that the asset management and filtering could also be moved out of the template files and put into a Bundle from flask-assets in a seperate assets.py. I could open a new issue for this and work on it if these types of changes seem reasonable.

[redshiftzero]

Actually, what about we just minify the CSS during the deb package build process, and ship the minified CSS along with the regular CSS? That's one less dependency for the app server

[heartsucker]

sass has a flag to minify and strip source maps, and it looks like we do that already according to grep:

install_files/ansible-base/roles/build-securedrop-app-code-deb-pkg/tasks/sass.yml
2:- name: Install sass Ruby gem
4:    name: sass
25:  shell: "/usr/local/bin/sass --force --stop-on-error --update sass:{{ securedrop_code_filtered }}/static/css --style compressed --sourcemap=none"
35:    - "sass"
36:    - ".sass-cache"

This may be closeable simply by removing it from the requirements files.