[MM-11860]: Expose slack attachment parsing functions in the model package (#9351)

Refactored parseSlackAttachment functions from https://github.com/mattermost/mattermost-server/blob/master/app/post.go#L312
into model/slack_attachments.go so that plugins have access to them.

model/slack_attachment.go

@@ -5,8 +5,11 @@ package model
 
 import (
 	"fmt"
+	"regexp"
 )
 
+var linkWithTextRegex = regexp.MustCompile(`<([^<\|]+)\|([^>]+)>`)
+
 type SlackAttachment struct {
 	Id         int64                   `json:"id"`
 	Fallback   string                  `json:"fallback"`
@@ -57,3 +60,25 @@ func StringifySlackFieldValue(a []*SlackAttachment) []*SlackAttachment {
 	}
 	return nonNilAttachments
 }
+
+// This method only parses and processes the attachments,
+// all else should be set in the post which is passed
+func ParseSlackAttachment(post *Post, attachments []*SlackAttachment) {
+	post.Type = POST_SLACK_ATTACHMENT
+
+	for _, attachment := range attachments {
+		attachment.Text = ParseSlackLinksToMarkdown(attachment.Text)
+		attachment.Pretext = ParseSlackLinksToMarkdown(attachment.Pretext)
+
+		for _, field := range attachment.Fields {
+			if value, ok := field.Value.(string); ok {
+				field.Value = ParseSlackLinksToMarkdown(value)
+			}
+		}
+	}
+	post.AddProp("attachments", attachments)
+}
+
+func ParseSlackLinksToMarkdown(text string) string {
+	return linkWithTextRegex.ReplaceAllString(text, "[${2}](${1})")
+}