Send email notification when username changed (#3022)

templates/username_change_body.html

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+{{define "username_change_body"}}
+
+<table align="center" border="0" cellpadding="0" cellspacing="0" width="100%" style="margin-top: 20px; line-height: 1.7; color: #555;">
+    <tr>
+        <td>
+            <table align="center" border="0" cellpadding="0" cellspacing="0" width="100%" style="max-width: 660px; font-family: Helvetica, Arial, sans-serif; font-size: 14px; background: #FFF;">
+                <tr>
+                    <td style="border: 1px solid #ddd;">
+                        <table align="center" border="0" cellpadding="0" cellspacing="0" width="100%" style="border-collapse: collapse;">
+                            <tr>
+                                <td style="padding: 20px 20px 10px; text-align:left;">
+                                    <img src="{{.Props.SiteURL}}/static/images/logo-email.png" width="130px" style="opacity: 0.5" alt="">
+                                </td>
+                            </tr>
+                            <tr>
+                                <td>
+                                    <table border="0" cellpadding="0" cellspacing="0" style="padding: 20px 50px 0; text-align: center; margin: 0 auto">
+                                        <tr>
+                                            <td style="border-bottom: 1px solid #ddd; padding: 0 0 20px;">
+                                                <h2 style="font-weight: normal; margin-top: 10px;">{{.Props.Title}}</h2>
+                                                <p>{{.Html.Info}}</p>
+                                            </td>
+                                        </tr>
+                                        <tr>
+                                            {{template "email_info" . }}
+                                        </tr>
+                                    </table>
+                                </td>
+                            </tr>
+                            <tr>
+                                {{template "email_footer" . }}
+                            </tr>
+                        </table>
+                    </td>
+                </tr>
+            </table>
+        </td>
+    </tr>
+</table>
+
+{{end}}

templates/username_change_subject.html

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+{{define "username_change_subject"}}[{{.Props.SiteName}}] {{.Props.Subject}}{{end}}