docs/website: move sponsors of past events to "past sponsors"

In this commit, we:

 - move the sponsors of the Buildroot Meeting at ELCE 2018 to "Past
   Sponsors"

 - move Scaleway, as a sponsor of Hackathon in Paris in 2018 to "Past
   Sponsors"

 - merge the Free Electrons and Bootlin entries together in "Past
   Sponsors"

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>