Album cover served in http (#264)

Apache is now serving album covers in https
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Two issues are addressed here. The first one was about Django replying with
mixed content (http) when queried for covers. Setting up the `X-Forwarded-Proto`
allows Django to know that the client is using https, and that the reply must
be https as well.

Second issue was a problem of permission causing Apache a denied access to
album cover folder. It was solved by adding another block for this path in
the Apache configuration file for funkwhale.

Here is how to modify your `funkwhale.conf` :

<VirtualHost *:443>

    ...
    Include /etc/letsencrypt/options-ssl-apache.conf

    #Add this new line
    RequestHeader set X-Forwarded-Proto "https"

    ...

    #Add this new block below the other <Directory/> blocks
    <Directory /srv/funkwhale/data/media/albums>
      Options FollowSymLinks
      AllowOverride None
      Require all granted
    </Directory>

    ...
</VirtualHost>