feat: Add a webpage for threepid validation links

src/service/templates/mail/change_email.txt

@@ -1,4 +1,4 @@
-{% extends "_base.txt.j2" %}
+{% extends "_base.txt" %}
 
 {% block content -%}
 Hello!

src/service/templates/mail/new_account.txt

@@ -1,4 +1,4 @@
-{% extends "_base.txt.j2" %}
+{% extends "_base.txt" %}
 
 {% block content -%}
 Hello!

src/service/templates/mail/password_reset.txt

@@ -1,4 +1,4 @@
-{% extends "_base.txt.j2" %}
+{% extends "_base.txt" %}
 
 {% block content -%}
 {%- if let Some(display_name) = display_name -%}

src/service/templates/mail/test.txt

@@ -1,4 +1,4 @@
-{% extends "_base.txt.j2" %}
+{% extends "_base.txt" %}
 
 {% block content -%}
 If you're seeing this, SMTP is configured correctly. :3