* Creates conf.d in /etc/prosody if missing.

Fixes a problem installing prosody 0.10 when using prosody repositories.

* Cleanups certificates on purge.

There are various occasions where users purge packages and the new installations after that generate certificate which doesn't end up in the java trust store on the target machine.

* Generate jicofo user and component passwords if missing.

There are situations where if prosody is already installed, the order of configuring the packages is not in the correct order. In those situations jitsi-meet-prosody got configured before jicofo and the user password and the component secret are not available and we ask the user for that and later when jicofo is configured we generate new set of them. Now we will end up always generating them in jitsi-meet-prosody or jicofo and we will reuse them. See https://github.com/jitsi/jicofo/pull/283.

Now the web app can be installed separately from jicofo and jitsi-videobridge, or can be installed on machine running nginx or apache. Currently only nginx will be configured and apache config is left to user. Later we can add and the apache config. Renames jitsi-meet to jitsi-meet-web (just the web content) and jitsi-meet-web-config (configuring jetty, nginx or apache). A new jitsi-meet package will be introduced, a meta package depending on jicofo, jvb and the current packages.