Move some variables out of model (#17909)

* Move some variables out of model

There were some stuff that's not necessary to be in model
and can just remain unexported variables.

```release-note
NONE
```

* Add license

```release-note
NONE
```

app/post.go

@@ -9,6 +9,7 @@ import (
 	"errors"
 	"fmt"
 	"net/http"
+	"regexp"
 	"strings"
 	"sync"
 	"time"
@@ -29,6 +30,8 @@ const (
 	PageDefault             = 0
 )
 
+var atMentionPattern = regexp.MustCompile(`\B@`)
+
 func (a *App) CreatePostAsUser(c *request.Context, post *model.Post, currentSessionId string, setOnline bool) (*model.Post, *model.AppError) {
 	// Check that channel has not been deleted
 	channel, errCh := a.Srv().Store.Channel().Get(post.ChannelId, true)
@@ -442,7 +445,7 @@ func (a *App) FillInPostProps(post *model.Post, channel *model.Channel) *model.A
 		post.DelProp("channel_mentions")
 	}
 
-	matched := model.AtMentionPattern.MatchString(post.Message)
+	matched := atMentionPattern.MatchString(post.Message)
 	if a.Srv().License() != nil && *a.Srv().License().Features.LDAPGroups && matched && !a.HasPermissionToChannel(post.UserId, post.ChannelId, model.PermissionUseGroupMentions) {
 		post.AddProp(model.PostPropsGroupHighlightDisabled, true)
 	}