Make attachment ids available in string form

Since JS can't handle 8 byte longs (!)

// FREEBIE

src/main/java/org/whispersystems/textsecuregcm/entities/AttachmentDescriptor.java

@@ -23,11 +23,15 @@ public class AttachmentDescriptor {
   @JsonProperty
   private long id;
 
+  @JsonProperty
+  private String idString;
+
   @JsonProperty
   private String location;
 
   public AttachmentDescriptor(long id, String location) {
     this.id       = id;
+    this.idString = String.valueOf(id);
     this.location = location;
   }
 
@@ -40,4 +44,8 @@ public class AttachmentDescriptor {
   public String getLocation() {
     return location;
   }
+
+  public String getIdString() {
+    return idString;
+  }
 }