Use mattermost v5 module (#1192)

vendor/github.com/mattermost/mattermost-server/v5/model/channel_mentions.go

@@ -0,0 +1,28 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package model
+
+import (
+	"regexp"
+	"strings"
+)
+
+var channelMentionRegexp = regexp.MustCompile(`\B~[a-zA-Z0-9\-_]+`)
+
+func ChannelMentions(message string) []string {
+	var names []string
+
+	if strings.Contains(message, "~") {
+		alreadyMentioned := make(map[string]bool)
+		for _, match := range channelMentionRegexp.FindAllString(message, -1) {
+			name := match[1:]
+			if !alreadyMentioned[name] {
+				names = append(names, name)
+				alreadyMentioned[name] = true
+			}
+		}
+	}
+
+	return names
+}