lib/connections, lib/model, gui: Specify allowed networks per device (fixes #219)

This adds a new config AllowedNetworks per device, which when set should
contain a list of network prefixes (192.168.0.0/126 etc) that are
allowed for the given device. The connection service will not attempt
connections to addresses outside of the given networks and incoming
connections will be rejected as well.

I've added the config to the normal device editor and shown it (when
set) in the device summary on the main screen.

There's a unit test for the IsAllowedNetwork method, I've done some
manual sanity testing on top of that.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4073

gui/default/assets/lang/lang-en.json

@@ -18,6 +18,7 @@
    "Advanced settings": "Advanced settings",
    "All Data": "All Data",
    "Allow Anonymous Usage Reporting?": "Allow Anonymous Usage Reporting?",
+   "Allowed Networks": "Allowed Networks",
    "Alphabetic": "Alphabetic",
    "An external command handles the versioning. It has to remove the file from the shared folder.": "An external command handles the versioning. It has to remove the file from the shared folder.",
    "An external command handles the versioning. It has to remove the file from the synced folder.": "An external command handles the versioning. It has to remove the file from the synced folder.",

gui/default/index.html

@@ -593,6 +593,12 @@
                       <th><span class="fa fa-fw fa-warning text-danger"></span>&nbsp;<span translate>Connection Type</span></th>
                       <td class="text-right">{{connections[deviceCfg.deviceID].type}}</td>
                     </tr>
+                    <tr ng-if="deviceCfg.allowedNetworks">
+                      <th><span class="fa fa-fw fa-filter"></span>&nbsp;<span translate>Allowed Networks</span></th>
+                      <td class="text-right">
+                        <span>{{deviceCfg.allowedNetworks.join(", ")}}</span>
+                      </td>
+                    </tr>
                     <tr ng-if="deviceCfg.compression != 'metadata'">
                       <th><span class="fa fa-fw fa-compress"></span>&nbsp;<span translate>Compression</span></th>
                       <td class="text-right">