Verify .ssh directory permissions before adding authkeys

[lavafroth]

Description of Changes

Addresses #260. The Linux implant module for authorized keys now checks the permissions on the .ssh directory before adding the keys. If the .ssh directory does not exist, besides creating it, we also record that fact for later reverts. Similar reverts can be made to the authorized_keys file if created and any permission changes.

Please note any noqa: comments needed to appease flake8.

Major Changes Implemented:

• Added tamper class ModifiedPermissions
• Authorized keys implant now checks the permissions on the .ssh directory
• Directory and file creation as well as permission changes by the implant can be reverted.

Pre-Merge Tasks

• Formatted all modified files w/ python-black
• Sorted imports for modified files w/ isort
• Ran flake8 on repo, and fixed any new problems w/ modified files
• Ran pytest test cases
• Added brief summary of updates to CHANGELOG (under [Unreleased])

For issues with pre-merge tasks, see CONTRIBUTING.md