[IMP] l10n_pl: some corrections in Polish CoA

PLN as default currency
Many improvements in taxes and fiscal positions
Fixes #825
[ADD] web_pdf_viewer: PDF in-browser preview

This module extends web client and reporting tool in order to allow a
preview of PDF files directly in the browser (if supported by the
browser). If you disable it, standard file download stays available.
[FIX] google_calendar: browse user as SUPERUSER_ID to be able to write/update google_calendar info on user
[FIX] web_tip: scroll if needed prior to displaying a tip
[IMP] sale,purchase: allow invoicing

(closes #500)

(cherry picked from commit 84c0595aef3a417702c81b5226ae1f61817a2bf8)
[FIX] sale: move menu to avoid error at install of website_quote

(closes #2728)
(cherry picked from commit 2b1e1e70dd537f02f776e58c9def195ecff839c5)
[IMP]Website Blog: Make Onboarding tour user friendly and change customize cover feature