Piwik URL should be auto-detected and cached in DB

[mattab]

In some cases, Piwik_Url will not return the current Piwik URL, for example when running from shell/cron sometimes the value is not available.

So far it hasn't been a problem so much as only the From: email would use the domain name. See faq: http://piwik.org/faq/troubleshooting/#faq_110

however, with Email reports coming soon, they will need to know the URL to link the icons / Piwik logo to the piwik server generating the reports.

Proposal:

• during install/update, when we are not in CLI mode, we could record in the config file, the current Piwik URL (fetched with Piwik_Url)
• Piwik_Url would then use this value if it is found in the config file
  • we could deprecate [mail] defaultHostnameIfEmpty
  • new setting would be [General] piwik_hostname

[mattab]

(In [4552]) Fixes #2318, #2325 - URL will be checked for in each piwik request to UI and API (not tracker), and will be set if not already set.
then, we use this URL for the HTML email images & link to the piwik reports