[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.

app/post.go

@@ -13,7 +13,6 @@ import (
 	"net/http"
 	"net/url"
 	"path"
-	"regexp"
 	"strings"
 
 	"github.com/dyatlov/go-opengraph/opengraph"
@@ -25,8 +24,6 @@ import (
 	"golang.org/x/net/html/charset"
 )
 
-var linkWithTextRegex = regexp.MustCompile(`<([^<\|]+)\|([^>]+)>`)
-
 func (a *App) CreatePostAsUser(post *model.Post) (*model.Post, *model.AppError) {
 	// Check that channel has not been deleted
 	var channel *model.Channel
@@ -308,28 +305,6 @@ func (a *App) handlePostEvents(post *model.Post, user *model.User, channel *mode
 	return nil
 }
 
-// This method only parses and processes the attachments,
-// all else should be set in the post which is passed
-func parseSlackAttachment(post *model.Post, attachments []*model.SlackAttachment) {
-	post.Type = model.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})")
-}
-
 func (a *App) SendEphemeralPost(userId string, post *model.Post) *model.Post {
 	post.Type = model.POST_EPHEMERAL
 
@@ -931,7 +906,7 @@ func (a *App) DoPostAction(postId, actionId, userId, selectedOption string) *mod
 
 	if response.EphemeralText != "" {
 		ephemeralPost := &model.Post{}
-		ephemeralPost.Message = parseSlackLinksToMarkdown(response.EphemeralText)
+		ephemeralPost.Message = model.ParseSlackLinksToMarkdown(response.EphemeralText)
 		ephemeralPost.ChannelId = post.ChannelId
 		ephemeralPost.RootId = post.RootId
 		if ephemeralPost.RootId == "" {