Further Piwik.org => Matomo.org replacements

[tsteur]

Those are only some replacements. Further changes will be needed to made later such as:

• License Headers (will need a separate PR)
• Wording in translations

We also need to see how to handle things like

• cookie names eg piwik_auth
• URL parameter like piwik_campaign to also support matomo_campaign
• DB prefix like piwik_
• Marketplace URL parameters like "piwik_version"
• plugin.json parameters like require "piwik"

Those things will all be done in other PRs. This PR only updates some links from piwik.org => matomo.org and some mentions of Piwik to Matomo. Also puts widgets from "About Piwik" to "About Matomo".

I expect a lot of tests will needed to be fixed and updated.

refs #12420

[tsteur]

tests should be fine now.