Add an option for Agent Forwarding

I don't understand why this is on by default, let alone not
configurable. Agent forwarding is a significant security concern, and
buggy agent forwarding is the cause of a couple existing bugs.

terminus-ssh/src/api.ts

@@ -36,6 +36,7 @@ export interface SSHConnection {
     skipBanner?: boolean
     disableDynamicTitle?: boolean
     jumpHost?: string
+    agentForward?: boolean
 
     algorithms?: {[t: string]: string[]}
 }

terminus-ssh/src/components/editConnectionModal.component.pug

@@ -82,6 +82,11 @@
                         .title X11 forwarding
                     toggle([(ngModel)]='connection.x11')
 
+                .form-line
+                    .header
+                        .title Allow Agent Forwarding
+                    toggle([(ngModel)]='connection.agentForward')
+
                 .form-line
                     .header
                         .title Tab color

terminus-ssh/src/services/ssh.service.ts

@@ -195,7 +195,7 @@ export class SSHService {
                     privateKey: privateKey || undefined,
                     tryKeyboard: true,
                     agent: agent || undefined,
-                    agentForward: !!agent,
+                    agentForward: session.connection.agentForward && !!agent,
                     keepaliveInterval: session.connection.keepaliveInterval,
                     keepaliveCountMax: session.connection.keepaliveCountMax,
                     readyTimeout: session.connection.readyTimeout,