Fix product names

[lippserd]

This PR fixes incorrect spelling of our product names, i.e.

• Icinga 2 instead of Icinga2,
• Icinga Web instead of IcingaWeb2 / Icingaweb2, and
• Icinga DB instead of IcingaDB.

Please note that the transition from Icinga Web 2 to Icinga Web is an ongoing process and you will still find Icinga Web 2 here and, but new docs should use Icinga Web.

Additionally, I have corrected the use of Icinga2, referring to the entire Icinga ecosystem, to Icinga.

[lippserd]

I rebased.