Moving markdown to corelibs (#16916)

* Moving markdown to corelibs

* Fix golangci-lint problem

* Renaming corelibs to pkg

* Fixing golangci-lint

* Renaming from pkg to shared

* Fixing gofmt

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

model/post.go

@@ -14,7 +14,7 @@ import (
 	"sync"
 	"unicode/utf8"
 
-	"github.com/mattermost/mattermost-server/v5/utils/markdown"
+	"github.com/mattermost/mattermost-server/v5/shared/markdown"
 )
 
 const (