Make WEBROOT_REDIRECT configurable in setup

closes Mailu#860

setup/flavors/compose/mailu.env

@@ -110,7 +110,13 @@ COMPRESSION_LEVEL={{ compression_level }}
 ###################################
 
 # Path to redirect / to
+{% if webroot_redirect == '' %}
+# You did not set any webroot redirect URL.
+# We default to /webmail to start error-free, regardless if you have webmail configured
 WEBROOT_REDIRECT=/webmail
+{% else %}
+WEBROOT_REDIRECT={{ webroot_redirect }}
+{% endif %}
 
 # Path to the admin interface if enabled
 WEB_ADMIN={{ admin_path }}

setup/templates/steps/compose/02_services.html

@@ -7,7 +7,8 @@
 <p>A Webmail is a Web interface exposing an email client. Mailu webmails are
 bound to the internal IMAP and SMTP server for users to access their mailbox through
 the Web. By exposing a complex application such as a Webmail, you should be aware of
-the security implications caused by such an increase of attack surface.<p>
+the security implications caused by such an increase of attack surface.
+Remember to set Webroot-redirect above!</p>
 <div class="form-group">
   <label>Enable Web email client (and path to the Web email client)</label>
 <!--   <div class="radio"> -->

setup/templates/steps/config.html

@@ -82,6 +82,13 @@
     <input class="form-control" type="text" name="admin_path" id="admin_path" style="display: none">
 </div>
 
+<div class="form-group">
+    <label>Webroot (<code>/</code>) redirect</label>
+    <p>The root/base of your web-server should redirect somewhere: This can be an the admin-interface or — as default — the webmailer. Note that if you change the webmail path below, you need to change this too.</p>
+    <p>Examples: <code>/webmail</code>; <code>/admin</code></p>
+    <input class="form-control" type="text" name="webroot_redirect" id="webroot_redirect" value="/webmail">
+</div>
+
 <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
 
 <script type="text/javascript" src="{{ url_for('static', filename='render.js') }}"></script>

setup/templates/steps/stack/02_services.html

@@ -7,7 +7,8 @@
 <p>A Webmail is a Web interface exposing an email client. Mailu webmails are
 bound to the internal IMAP and SMTP server for users to access their mailbox through
 the Web. By exposing a complex application such as a Webmail, you should be aware of
-the security implications caused by such an increase of attack surface.<p>
+the security implications caused by such an increase of attack surface.
+Remember to set Webroot-redirect above!</p>
 <div class="form-group">
   <label>Enable Web email client (and path to the Web email client)</label>
   <br/>