Solves issue #37 (Pictures subfolder in past events) and #38 ( Presentations subfolder in past events)

FIrst part of "Set gray background color to the subscribe banner in Events and Home page but not in News page"

In upcoming events section, the width of the cards is not correct because the page structure is not identical in Plone than in the prototype

Add new field to generic event called register_url and hide Add calendar and Register buttons for past events

Solves #35 (Parametrize some fields of the google calendar entry..) but without parametrizing the time and date

Presentations title changed from h3 to h2 because the title font sizes have been normalized in the theme

Load presentations through Ajax (JS code was not being called) both in the modal and the ‘page to share’ windows