vcl.apache.org updates

[Andy Kurth <an...@ncsu.edu>]

I made a few minor changes to the project front page: http://vcl.apache.org/

We received a request from the ASF board to add an advertisement link to
ASF events.  There are a few other details the page wasn't in compliance
with per:
https://www.apache.org/foundation/marks/pmcs#navigation

Checked by this tool:
https://whimsy.apache.org/site/project/vcl

I did the following:
* Added the events ad at the bottom of the left nav column
* Renamed "Become a Sponsor" to "Sponsorship" to hopefully match the tool's
regex
* Changed the "License" link to point to the ASF's main license page
* Added the "Security" link

As of now, the tool isn't showing all green because I don't believe it has
checked the site since all the changes were made.  Hopefully it reports all
green the next time it runs.

Please reply if it doesn't look correct for you.

Thanks,
Andy