Adds optional param channel_id to TeachingEventAddAttendee (#1330)

Teams need to be able to attribute event sign-ups in the same way that they
do for mailing list and adviser with a query parameter in the format ?channel={channel_id}

Link to Trello card: 
https://trello.com/c/hEzzR5IP/5001-investigate-bam-channel-capture-for-events

* Adds optional param `channel_id` to TeachingEventAddAttendee
* Adds Unit test

GetIntoTeachingApi/Models/GetIntoTeaching/TeachingEventAddAttendee.cs

@@ -15,6 +15,10 @@ public class TeachingEventAddAttendee
 
         [SwaggerSchema(WriteOnly = true)]
         public Guid? EventId { get; set; }
+
+        [SwaggerSchema(WriteOnly = true)]
+        public int? ChannelId { get; set; }
+
         [SwaggerSchema(WriteOnly = true)]
         public Guid? AcceptedPolicyId { get; set; }
         public Guid? PreferredTeachingSubjectId { get; set; }
@@ -136,7 +140,7 @@ private void ConfigureChannel(Candidate candidate)
         {
             if (CandidateId == null)
             {
-                candidate.ChannelId = (int?)Candidate.Channel.Event;
+                candidate.ChannelId = ChannelId ?? (int?)Candidate.Channel.Event;
             }
         }
 

GetIntoTeachingApiTests/Models/GetIntoTeaching/TeachingEventAddAttendeeTests.cs

@@ -156,6 +156,14 @@ public void Candidate_ChannelIdWhenCandidateIdIsNull_IsMailingList()
             request.Candidate.ChannelId.Should().Be((int)Candidate.Channel.Event);
         }
 
+        [Fact]
+        public void Candidate_WithNonNullChannelIdWhenCandidateIdIsNull_IsMailingList()
+        {
+            var request = new TeachingEventAddAttendee() { CandidateId = null, ChannelId = 456 };
+
+            request.Candidate.ChannelId.Should().Be(456);
+        }
+
         [Fact]
         public void Candidate_ChannelIdWhenCandidateIdIsNotNull_IsNotChanged()
         {