Enable HTTPS for EmonCMS on Raspbian

Our EmonCMS installation at home currently runs only over HTTP, which is not secure at all. Enabling HTTPS is a must do. However, there's a small tweak needed for it to work at the AllowOverride option.
Tip: If you run into the issue where you have HTTPS enabled, but the logging in returns always the login screen with message 'undefined', follow the fix explained below.

Check if HTTPS is already enabled for your Apache installation:

root@raspi1:~# netstat -an | grep LISTEN
tcp        0      0 0.0.0.0:80              0.0.0.0:*               LISTEN
tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN

As you can see, there is nothing running on port 443 (HTTPS) yet, so we'll need to enable the HTTPS configuration for Apache.

Enable the default HTTPS (SSL) configuration for Apache:

1. Check if the SSL module is already enabled for Apache.

root@raspi1:~# ls -l /etc/apache2/mods-enabled/*ssl*
root@raspi1:~#

If nothing is returned, the module is not enabled yet and needs to be linked from /etc/apache2/mods-available/

2. Make a symbolic link for the SSL module.

root@raspi1:~# cd /etc/apache2/mods-enabled/
root@raspi1:~# ln -s ../mods-available/ssl.conf ssl.conf
root@raspi1:~# ln -s ../mods-available/ssl.load ssl.load

3. Make a symbolic link for the SSL config file.

root@raspi1:~# cd /etc/apache2/sites-enabled/
root@raspi1:~# ln -s ../sites-available/default-ssl 000-default-ssl

4. Change two lines in the config file.

        DocumentRoot /var/www
        <Directory />
                Options FollowSymLinks
                #Changed None to All on 18/11/2016
                AllowOverride All
        </Directory>
        <Directory /var/www/>
                Options Indexes FollowSymLinks MultiViews
                #Changed None to All on 18/11/2016
                AllowOverride All
                Order allow,deny
                allow from all
        </Directory>

5. Restart your Apache now.

root@raspi1:~# /etc/init.d/apache2 restart

This tip came from here.