email: HTML templates and multipart messages

Emails now render an HTML template from templates/email with a plaintext
fallback, sent as multipart/alternative. A configurable brand name shows
in the template. Reset and confirmation mails share one styled base.

src/email.rs

@@ -0,0 +1,56 @@
+// Email content: renders the HTML templates in ../templates/email and pairs each
+// with a plaintext fallback. The link layer sends both as multipart/alternative.
+
+pub struct Mail {
+    pub subject: String,
+    pub text: String,
+    pub html: String,
+}
+
+const BASE: &str = include_str!("../templates/email/base.html");
+
+fn render(brand: &str, title: &str, message: &str, code: &str, note: &str) -> String {
+    BASE.replace("{{brand}}", &escape(brand))
+        .replace("{{title}}", &escape(title))
+        .replace("{{message}}", &escape(message))
+        .replace("{{code}}", &escape(code))
+        .replace("{{note}}", &escape(note))
+}
+
+pub fn reset(brand: &str, account: &str, code: &str) -> Mail {
+    Mail {
+        subject: format!("Password reset for {account}"),
+        text: format!(
+            "Your password reset code for {account} is: {code}\nIt expires in 15 minutes.\nReset with:\n  /msg NickServ RESETPASS {account} {code} <newpassword>\n"
+        ),
+        html: render(
+            brand,
+            "Password reset",
+            &format!("Use this code to reset the password for your account {account}."),
+            code,
+            "This code expires in 15 minutes.",
+        ),
+    }
+}
+
+pub fn confirm(brand: &str, account: &str, code: &str) -> Mail {
+    Mail {
+        subject: format!("Confirm your {account} registration"),
+        text: format!(
+            "Confirm your account {account} with:\n  /msg NickServ CONFIRM {code}\nThe code expires in 15 minutes.\n"
+        ),
+        html: render(
+            brand,
+            "Confirm your account",
+            &format!("Welcome! Confirm the email for your account {account} with the code below."),
+            code,
+            "This code expires in 15 minutes.",
+        ),
+    }
+}
+
+// Escape the few characters that matter inside HTML text so a value can't break
+// out of the template.
+fn escape(s: &str) -> String {
+    s.replace('&', "&amp;").replace('<', "&lt;").replace('>', "&gt;")
+}