feat(feedback): show the sender's interface language and connector bot in admin

In the console feedback detail, show the sender's interface language (account
preferred_language) always, and — for a message that arrived through an external
connector (currently Telegram) — the bot they last used (en/ru, from the
account's service_language).

backend/internal/adminconsole/templates/pages/feedback_detail.gohtml

@@ -5,7 +5,8 @@
 <section class="panel"><h2>Message</h2>
 <ul class="kv">
 <li><b>From</b> <a href="/_gm/users/{{.AccountID}}">{{.SenderName}}</a> ({{.Source}})</li>
-<li><b>Channel</b> {{.Channel}}</li>
+<li><b>Channel</b> {{.Channel}}{{if .BotLanguage}} (bot: {{.BotLanguage}}){{end}}</li>
+<li><b>Interface language</b> {{.InterfaceLanguage}}</li>
 <li><b>IP</b> {{if .IP}}<code>{{.IP}}</code>{{else}}<span class="note">none</span>{{end}}</li>
 <li><b>Filed</b> {{.CreatedAt}}</li>
 <li><b>State</b> {{if .Archived}}archived{{else if .Read}}read{{else}}<span class="warn">unread</span>{{end}}</li>