PLT-4839 Split too-long Slack messages on import. (#4679)

* PLT-4839 Split too-long Slack messages on import.

This PR also takes the opportunity to make the max values for Post
properties into constants for easier use elsewhere, as has previously
been done for Channel properties.

* Only count runes once.

model/post.go

@@ -19,6 +19,11 @@ const (
 	POST_HEADER_CHANGE         = "system_header_change"
 	POST_CHANNEL_DELETED       = "system_channel_deleted"
 	POST_EPHEMERAL             = "system_ephemeral"
+	POST_FILEIDS_MAX_RUNES     = 150
+	POST_FILENAMES_MAX_RUNES   = 4000
+	POST_HASHTAGS_MAX_RUNES    = 1000
+	POST_MESSAGE_MAX_RUNES     = 4000
+	POST_PROPS_MAX_RUNES       = 8000
 )
 
 type Post struct {
@@ -103,11 +108,11 @@ func (o *Post) IsValid() *AppError {
 		return NewLocAppError("Post.IsValid", "model.post.is_valid.original_id.app_error", nil, "")
 	}
 
-	if utf8.RuneCountInString(o.Message) > 4000 {
+	if utf8.RuneCountInString(o.Message) > POST_MESSAGE_MAX_RUNES {
 		return NewLocAppError("Post.IsValid", "model.post.is_valid.msg.app_error", nil, "id="+o.Id)
 	}
 
-	if utf8.RuneCountInString(o.Hashtags) > 1000 {
+	if utf8.RuneCountInString(o.Hashtags) > POST_HASHTAGS_MAX_RUNES {
 		return NewLocAppError("Post.IsValid", "model.post.is_valid.hashtags.app_error", nil, "id="+o.Id)
 	}
 
@@ -116,15 +121,15 @@ func (o *Post) IsValid() *AppError {
 		return NewLocAppError("Post.IsValid", "model.post.is_valid.type.app_error", nil, "id="+o.Type)
 	}
 
-	if utf8.RuneCountInString(ArrayToJson(o.Filenames)) > 4000 {
+	if utf8.RuneCountInString(ArrayToJson(o.Filenames)) > POST_FILENAMES_MAX_RUNES {
 		return NewLocAppError("Post.IsValid", "model.post.is_valid.filenames.app_error", nil, "id="+o.Id)
 	}
 
-	if utf8.RuneCountInString(ArrayToJson(o.FileIds)) > 150 {
+	if utf8.RuneCountInString(ArrayToJson(o.FileIds)) > POST_FILEIDS_MAX_RUNES {
 		return NewLocAppError("Post.IsValid", "model.post.is_valid.file_ids.app_error", nil, "id="+o.Id)
 	}
 
-	if utf8.RuneCountInString(StringInterfaceToJson(o.Props)) > 8000 {
+	if utf8.RuneCountInString(StringInterfaceToJson(o.Props)) > POST_PROPS_MAX_RUNES {
 		return NewLocAppError("Post.IsValid", "model.post.is_valid.props.app_error", nil, "id="+o.Id)
 	}