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/templates/aria2.conf.j2

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+dir={{ aria2_download_dir }}
+bt-exclude-tracker=*
+bt-prioritize-piece=head,tail
+bt-require-crypto=true
+dht-listen-port={{ aria2_dht_listen_ports }}
+dht-entry-point=dht.transmissionbt.com:6881
+dht-file-path=/var/lib/aria2/dht.dat
+dht-file-path6=/var/lib/aria2/dht6.dat
+listen-port={{ aria2_listen_ports }}
+enable-rpc=true
+rpc-listen-all=true
+log=/var/log/aria2.log
+log-level=notice
+continue
+save-session=/var/lib/aria2/aria2.state
+input-file=/var/lib/aria2/aria2.state
+save-session-interval=10

roles/aria2/templates/configuration.js.j2

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+angular
+.module('webui.services.configuration',  [])
+.constant('$name', 'Aria2 WebUI')  // name used across the entire UI
+.constant('$titlePattern', 'active: {active} - waiting: {waiting} - stopped: {stopped} — {name}')
+.constant('$pageSize', 11)         // number of downloads shown before pagination kicks in
+.constant('$authconf', {           // default authentication configuration, never fill it in case the webui is hosted in public IP as it can be compromised
+  host: location.protocol.startsWith('http') ? location.hostname : 'localhost',
+  path: '/jsonrpc',
+  port: 6800,
+  encrypt: false,
+  auth: {                          // either add the token field or the user and pass field, not both.
+  // token: '$YOUR_SECRET_TOKEN$'
+  /*-----------------------------*/
+  // user: '*YOUR_USERNAME*',
+  // pass: '*YOUR_SECRET_PASS*'
+  },
+  directURL: ''                   // If supplied, links will be created to enable direct download from the aria2 server, requires appropriate webserver to be configured
+})
+.constant('$enable', {
+
+  torrent: true,  // bittorrent support only enabled if supported by aria2 build, set to false otherwise to permanently disable it
+
+  metalink: true, // metalink support only enabled if supported by aria2 build, set to false to permanently disable it
+
+  sidebar: {            // configuration related to the sidebar next to the list of downloads
+    show: true,         // set to false to completely hide the sidebar. Other elements inside will be automatically hidden
+
+    stats: true,        // set to false to hide the global statistic section (contains the speed graph for now)
+
+    filters: true,      // set to false to hide the  Download Filters
+
+    starredProps: true // only shown when at least one property is added to the starred list, set to false to permanently hide the Quick Access Settings inside the sidebar
+  }
+})
+.constant('$starredProps', [   // default list of Quick Access Properties. Can be overridden by making modification through the Global Settings dialog
+  // go to Global Settings dialog to see their description
+  'dir', 'conf-path', 'auto-file-renaming', 'max-connection-per-server'
+])
+.constant('$downloadProps', [ // Similar to starred Quick Access properties but for adding new downloads.
+  // go to Advance Download Options when adding a new download to view the list of possible options
+  'header', 'http-user', 'http-passwd', 'pause', 'dir', 'max-connection-per-server'
+])
+.constant('$globalTimeout', 1000)  // interval to update the individual downloads
+;
+
+