Bump Post.Props size limits (#18370)

app/webhook_test.go

@@ -511,8 +511,7 @@ func TestSplitWebhookPostAttachments(t *testing.T) {
 			post: makePost(maxPostSize*3/2, []int{5150, 2000, model.PostPropsMaxUserRunes - 1000}),
 			expected: []*model.Post{
 				makePost(maxPostSize, nil),
-				makePost(maxPostSize/2, []int{5150}),
+				makePost(maxPostSize/2, []int{5150, 2000}),
-				makePost(0, []int{2000}),
 				makePost(0, []int{model.PostPropsMaxUserRunes - 1000}),
 			},
 		},

model/post.go

@@ -54,8 +54,8 @@ const (
 	PostMessageMaxRunesV1 = 4000
 	PostMessageMaxBytesV2 = 65535                     // Maximum size of a TEXT column in MySQL
 	PostMessageMaxRunesV2 = PostMessageMaxBytesV2 / 4 // Assume a worst-case representation
-	PostPropsMaxRunes     = 8000
+	PostPropsMaxRunes     = 800000
-	PostPropsMaxUserRunes = PostPropsMaxRunes - 400 // Leave some room for system / pre-save modifications
+	PostPropsMaxUserRunes = PostPropsMaxRunes - 40000 // Leave some room for system / pre-save modifications
 
 	PropsAddChannelMember = "add_channel_member"