Create RPM File

[elwerene]

I added configurations for https://crates.io/crates/cargo-rpm to create a rpm package.

Installation:

• user/group alexandrie is created if it doesn't yet exist
• binary is installed to /usr/bin/alexandrie
• readonly resources are installed to /usr/share/alexandrie
• configuration file is installed to /etc/alexandrie.conf
• a systemd service is installed which starts alexandrie with /etc/alexandrie.conf
• storage (with write permissions for user alexandrie) is set up with empty directories in /var/db/alexandrie
• if there's not yet a git repository in /var/db/alexandrie/crate-index, it is setup with the default config.json of the installation script