Add rpm import in %post

[rocodes]

Add rpm key import in boostrap package post. There are 3 cases (clean install, upgrade package, uninstall); cover all of them. In a clean install scenario, import the key to the rpm database. In a package upgrade scenario (key bump), remove the old key and import the key again - this may not be needed if upstream changes are incorporated into a version of rpm available in dom0. In uninstall case, remove the key from the rpm database after the package is uninstalled.

See https://docs.fedoraproject.org/en-US/packaging-guidelines/Scriptlets/ for guidelines on scriptlet ordering in upgrade, uninstall, etc.

Fixes freedomofpress/securedrop-workstation#423

Test plan

• Visual review
• CI (in draft until CI Add github actions, license file, security.md #6 and rebase)
• Clean install: build this package with make build-rpm. Package installs successfully; repo file is present in /etc/yum.repos.d and rpm -q gpg-pubkey --qf '%{NAME}-%{VERSION}-%{RELEASE}\t%{SUMMARY}\n' | grep 'SecureDrop Release Signing Key' shows an entry corresponding to our release signing key in the rpm db.
• Upgrade testing: TODO. This will require an upgrade test plan.