v8.0 module release (#22975)

https://mattermost.atlassian.net/browse/MM-52079

```release-note
We upgrade the module version to 8.0. The new module path is github.com/mattermost-server/server/v8.
```


Co-authored-by: Doug Lauder <wiggin77@warpmail.net>

server/model/mention_map.go

@@ -0,0 +1,80 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package model
+
+import (
+	"fmt"
+	"net/url"
+)
+
+type UserMentionMap map[string]string
+type ChannelMentionMap map[string]string
+
+const (
+	userMentionsKey       = "user_mentions"
+	userMentionsIdsKey    = "user_mentions_ids"
+	channelMentionsKey    = "channel_mentions"
+	channelMentionsIdsKey = "channel_mentions_ids"
+)
+
+func UserMentionMapFromURLValues(values url.Values) (UserMentionMap, error) {
+	return mentionsFromURLValues(values, userMentionsKey, userMentionsIdsKey)
+}
+
+func (m UserMentionMap) ToURLValues() url.Values {
+	return mentionsToURLValues(m, userMentionsKey, userMentionsIdsKey)
+}
+
+func ChannelMentionMapFromURLValues(values url.Values) (ChannelMentionMap, error) {
+	return mentionsFromURLValues(values, channelMentionsKey, channelMentionsIdsKey)
+}
+
+func (m ChannelMentionMap) ToURLValues() url.Values {
+	return mentionsToURLValues(m, channelMentionsKey, channelMentionsIdsKey)
+}
+
+func mentionsFromURLValues(values url.Values, mentionKey, idKey string) (map[string]string, error) {
+	mentions, mentionsOk := values[mentionKey]
+	ids, idsOk := values[idKey]
+
+	if !mentionsOk && !idsOk {
+		return map[string]string{}, nil
+	}
+
+	if !mentionsOk {
+		return nil, fmt.Errorf("%s key not found", mentionKey)
+	}
+
+	if !idsOk {
+		return nil, fmt.Errorf("%s key not found", idKey)
+	}
+
+	if len(mentions) != len(ids) {
+		return nil, fmt.Errorf("keys %s and %s have different length", mentionKey, idKey)
+	}
+
+	mentionsMap := make(map[string]string)
+	for i, mention := range mentions {
+		id := ids[i]
+
+		if oldId, ok := mentionsMap[mention]; ok && oldId != id {
+			return nil, fmt.Errorf("key %s has two different values: %s and %s", mention, oldId, id)
+		}
+
+		mentionsMap[mention] = id
+	}
+
+	return mentionsMap, nil
+}
+
+func mentionsToURLValues(mentions map[string]string, mentionKey, idKey string) url.Values {
+	values := url.Values{}
+
+	for mention, id := range mentions {
+		values.Add(mentionKey, mention)
+		values.Add(idKey, id)
+	}
+
+	return values
+}