Ensure users are active on the change when adding to attention set

Until now, it was technically possible to add users to the attention set
even if they had no interactions on the change. This is a minor bug
since the frontend took care of this even if such a thing happened, and
also it required users to manually do it using the REST API.

This change ensures users can't be added to the attention set if they
are not active (reviewer, cc, owner, uploader) on the change.

On reply, we don't throw an error when trying to add an inactive user
(but rather just ignore it in ChangeUpdate). The reason for this is the
fact it's complicated to calculate the current reviewers since on reply
we can also add new reviewers (which would require "resolving" all
new reviewers unnecessarily, which is a latency concern, In
ChangeUpdate it's done anyway so we can check it there).

Tests that allowed adding to the attention set when the user was not
active in the change had to be updated.

Change-Id: Icd779ce3b4c766a1a941680cde5d2b87d055b553
diff --git a/Documentation/rest-api-changes.txt b/Documentation/rest-api-changes.txt
index 6b73482..c74b2c3 100644
--- a/Documentation/rest-api-changes.txt
+++ b/Documentation/rest-api-changes.txt
@@ -5935,6 +5935,8 @@
 If a user is added while already in the attention set, the
 request is silently ignored.
 
+The user must be a reviewer, cc, uploader, or owner on the change.
+
 .Request
 ----
   POST /changes/myProject~master~I8473b95934b5732ac55d26311a706c9c2bde9940/attention HTTP/1.0
@@ -7662,7 +7664,8 @@
 `ready` and `work_in_progress` to be true.
 |`add_to_attention_set`                |optional|
 list of link:#attention-set-input[AttentionSetInput] entities to add
-to the link:#attention-set[attention set].
+to the link:#attention-set[attention set]. Users that are not reviewers,
+ccs, owner, or uploader are silently ignored.
 |`remove_from_attention_set`           |optional|
 list of link:#attention-set-input[AttentionSetInput] entities to remove
 from the link:#attention-set[attention set].