aria2: Deploy aria2 download manager

The *aria2* role installs the *aria2* download manager and sets it up to
run as a system service with RPC enabled. It also sets up the web UI,
though that must be installed manually from an archive, for now.

roles/aria2/files/aria2.httpd.conf

@@ -0,0 +1,5 @@
+Alias /aria2 /var/lib/aria2/webui
+
+<Location /aria2>
+	Require all granted
+</Location>

roles/aria2/files/aria2.service

@@ -0,0 +1,11 @@
+[Unit]
+Description=aria2 Download Manager
+
+[Service]
+Type=simple
+User=aria2
+Group=aria2
+ExecStart=/usr/bin/aria2c --daemon=false --conf-path=/etc/aria2.conf --log=-
+
+[Install]
+WantedBy=multi-user.target