[MM-14846] Update EditAt for FileIds and Attachment in Post + Ignore FileIds Updates (#10540)

* Set EditAt for FileIds and Attachments; Disallow update/patch of FileIds in API Handler

* Add custom comparison methods for StringArray and Post Attachments

* gofmt

* Split up comparison function to child structs

* Naming consistency

* gofmt

model/post.go

@@ -403,6 +403,23 @@ func (o *Post) Attachments() []*SlackAttachment {
 	return ret
 }
 
+func (o *Post) AttachmentsEqual(input *Post) bool {
+	attachments := o.Attachments()
+	inputAttachments := input.Attachments()
+
+	if len(attachments) != len(inputAttachments) {
+		return false
+	}
+
+	for i := range attachments {
+		if !attachments[i].Equals(inputAttachments[i]) {
+			return false
+		}
+	}
+
+	return true
+}
+
 var markdownDestinationEscaper = strings.NewReplacer(
 	`\`, `\\`,
 	`<`, `\<`,