deploy.sh: More fixes

[siddhpant]

• Root is needed for ./src/check_config.py too in the end. Thus, mandate running deploy.sh with root, and remove all sudo uses.
• Use apt-get instead of apt, since latter doesn't have stable CLI interface.
• Separate eval from if statement to correctly check return code.
• Remove /usr/bin/slimbookbattery* symlinks when clearing up previous installation.
• Exit on error when doing Slimbook Battery installation.
• Do not make changelog folder as it will be a file.
• Make custom and default folders.

[siddhpant]

CI fails because 3.6 is not there in ubuntu latest. 3.6 is no longer supported, EOL was a year ago.