Make matomo.js and matomo.php the default Tracking API endpoints used

[mattab]

As part of our ongoing to #12420 renaming of everything from Piwik to Matomo, the next most important step is that our JS Tracking code should reference matomo.js and matomo.php instead of piwik.js|php. This remaining use of "Piwik" seems to be the last highly visible mention left in the product.

Suggested steps:

• The tracking code in Administration > Tracking code should reference matomo.js|php
• The API output for SitesManager.getJavascriptTag and others like SitesManager.getImageTrackingCode should also reference matomo.js|php
• it's very important that we keep backward compatibility, so the file "piwik.js" should still be there and be a copy of matomo.js file (we will need to update instructions on how to build the compiled matomo.js in https://github.com/matomo-org/matomo/blob/master/js/README.md#deployment
• And add a ReleaseChecklist test that the matomo.js is the same as piwik.js
• Search and check/replace all occurences of piwik.js in the codebase (there are many)
• Ideally we also rename the CustomPiwikJs plugin to CustomMatomoJs or CustomTrackerJs
• Possibly other steps I missed

[tsteur]

• Developer docs need to be adjust everywhere and mention piwik.js or matomo.js
• It can ONLY be changed for new installs, otherwise this is a breaking change as many projects and hosters might block anything that does not go to index.php or piwik.php such as the popular nginx configuration: https://github.com/perusio/piwik-nginx/blob/master/apps/piwik/piwik.conf . Therefore we need to make sure in API output it is only returned for new installs.
• This needs to be also announced in advance as otherwise can cause failures where people handle piwik.js in a custom way like we do.
• Tests need to use matomo.js
• A mechanism needs to be there that also copies js/matomo.js to js/piwik.js and needs to be tested they are the same in ReleaseCheckList test
• the global Matomo will need to be defined within piwik.js same as Piwik and this may needs announcement as well.

[tsteur]

fyi this issue will also need matomo-org/matomo-log-analytics#224