Improve system admin permissions to dms and gms (#29656)

* revert the api4 portion of 57372bd06b

* test: Add system admin channel access tests

* skip a.HasPermissionToTeam for channels without a team

* fix TestPostGetInfo

server/channels/api4/post_test.go

@@ -1995,13 +1995,39 @@ func TestGetPostsForChannel(t *testing.T) {
 		CheckOKStatus(t, resp)
 		require.Len(t, posts.Order, 10, "expected 10 posts")
 
-		// allow viewing of direct messages
+		// System admin can access public channel without being member
+		adminPublicChannel := th.CreatePublicChannel()
+		th.CreateMessagePostNoClient(adminPublicChannel, "admin channel post", model.GetMillis())
+		posts, resp, err = c.GetPostsForChannel(context.Background(), adminPublicChannel.Id, 0, 100, "", false, false)
+		require.NoError(t, err)
+		CheckOKStatus(t, resp)
+		require.NotEmpty(t, posts.Order)
+
+		// System admin can access private channel without being member
+		privateChannel := th.CreatePrivateChannel()
+		th.CreateMessagePostNoClient(privateChannel, "private channel post", model.GetMillis())
+		posts, resp, err = c.GetPostsForChannel(context.Background(), privateChannel.Id, 0, 100, "", false, false)
+		require.NoError(t, err)
+		CheckOKStatus(t, resp)
+		require.NotEmpty(t, posts.Order)
+
+		// System admin can access direct messages without being member
 		dmChannel := th.CreateDmChannel(th.BasicUser2)
 		th.CreateMessagePostNoClient(dmChannel, "test1", model.GetMillis())
-
 		posts, resp, err = c.GetPostsForChannel(context.Background(), dmChannel.Id, 0, 100, "", false, false)
 		require.NoError(t, err)
 		CheckOKStatus(t, resp)
+		require.NotEmpty(t, posts.Order)
+
+		// System admin can access group messages without being member
+		user3 := th.CreateUser()
+		gmChannel, _, err := th.Client.CreateGroupChannel(context.Background(), []string{th.BasicUser.Id, th.BasicUser2.Id, user3.Id})
+		require.NoError(t, err)
+		th.CreateMessagePostNoClient(gmChannel, "test2", model.GetMillis())
+		posts, resp, err = c.GetPostsForChannel(context.Background(), gmChannel.Id, 0, 100, "", false, false)
+		require.NoError(t, err)
+		CheckOKStatus(t, resp)
+		require.NotEmpty(t, posts.Order)
 	})
 }
 
@@ -4544,7 +4570,7 @@ func TestPostGetInfo(t *testing.T) {
 			channel:   gmChannel,
 			post:      gmPost,
 			client:    sysadminClient,
-			hasAccess: false,
+			hasAccess: true,
 		},
 
 		// DM channel
@@ -4563,7 +4589,7 @@ func TestPostGetInfo(t *testing.T) {
 			channel:   dmChannel,
 			post:      dmPost,
 			client:    sysadminClient,
-			hasAccess: false,
+			hasAccess: true,
 		},
 
 		// Open channel - Open Team