Make install.sh work with RHEL-based distros

[egg82]

Similar to #2046

Changes made:

• Uses /etc/os-release to determine OS type
• Uses command -v dnf to determine if dnf is available, and falls back to yum if not
• Uses yum/dnf in place of apt and apt-get where applicable
• Changes support (unsupported) wording
• Changes locale wording when in a RHEL environment
• Trades apt keyrings for yum GPG keys where applicable
• Slightly changes postgres install and initialization (and service name) for RHEL-based systems
• Changes www-data for nginx user/group where applicable
• Installs pango, pango-devel, cairo, cairo-devel, and libffi-devel from proper sources when in a RHEL environment
• Uses pyenv to install proper python version when in RHEL environment
• Uses proper nodejs setup script and dnf/yum install when in a RHEL environment
• Changes python manage.py to python3.11 manage.py to be consistent
• Changes /etc/nginx/sites-available to /etc/nginx/conf.d when in a RHEL environment
• Fixes the directory permissions of all of the parent directories for /rmm/api/tacticalrmm/tacticalrmm.sock (nginx can't access socket)

Additionally, the contents of /rmm/api/tacticalrmm/tacticalrmm/local_settings.py ($local_settings or $localvars) to include CORS_ALLOW_HEADERS and CORS_ALLOW_METHODS because otherwise Firefox refuses to load anything and breaks.

The biggest change, as earlier, is the use of pyenv.