Navigation menu: clean up

some ideas picked up here and there, to be completed:

• "Upcoming events", somehow interesting info, but landing on this indico page is not so nice
• look at top 20 of hifis.net visits and make them reachable in 1 click
• simplify "about HIFIS" section, look at other platforms websites
• avoid "subscribe to" and "further information" in the menu, move to... ?

cc. for completion / food for thought @jandt-desy @SpickerHZB @erxleb87