Update social media header and logo pictures

[francoisgoupil]

I noticed that for instance the scikit-learn logo of our twitter account is not the right one.
Also, we may want to change the cover banner of this account.

[francoisgoupil]

Note: I don't have access to the admin interface I only have posting rights through tweetdeck.

[laurburke]

@francoisgoupil What would you suggest as a cover banner? We added this image last year because it was previously empty. I can design an element if you have suggestions for the content!

[laurburke]

The Twitter logo has been updated. Some social media accounts will show a black/gray background for profile images with a transparent background. Here is the image I used, where I centered it on a white background.