[MM-16389] Add icon_emoji parameter to webhooks (#11586)

[MM-16389] Add icon_emoji field to posts from incoming webhooks

To be used to substitute the profile picture on posts.

app/post_metadata_test.go

@@ -235,6 +235,49 @@ func TestPreparePostForClient(t *testing.T) {
 		})
 	})
 
+	t.Run("emojis overriding profile icon", func(t *testing.T) {
+		th := setup()
+		defer th.TearDown()
+
+		prepare := func(override bool, url, emoji string) *model.Post {
+			th.App.UpdateConfig(func(cfg *model.Config) {
+				*cfg.ServiceSettings.EnablePostIconOverride = override
+			})
+
+			post, err := th.App.CreatePost(&model.Post{
+				UserId:    th.BasicUser.Id,
+				ChannelId: th.BasicChannel.Id,
+				Message:   "Test",
+			}, th.BasicChannel, false)
+
+			require.Nil(t, err)
+
+			post.AddProp(model.POST_PROPS_OVERRIDE_ICON_URL, url)
+			post.AddProp(model.POST_PROPS_OVERRIDE_ICON_EMOJI, emoji)
+
+			return th.App.PreparePostForClient(post, false, false)
+		}
+
+		emoji := "basketball"
+		url := "http://host.com/image.png"
+		overridenUrl := "/static/emoji/1f3c0.png"
+
+		t.Run("does not override icon URL", func(t *testing.T) {
+			clientPost := prepare(false, url, emoji)
+
+			s, _ := clientPost.Props[model.POST_PROPS_OVERRIDE_ICON_URL]
+			assert.EqualValues(t, url, s)
+		})
+
+		t.Run("overrides icon URL", func(t *testing.T) {
+			clientPost := prepare(true, url, emoji)
+
+			s, _ := clientPost.Props[model.POST_PROPS_OVERRIDE_ICON_URL]
+			assert.EqualValues(t, overridenUrl, s)
+		})
+
+	})
+
 	t.Run("markdown image dimensions", func(t *testing.T) {
 		th := setup()
 		defer th.TearDown()