Add thumbnail logo to competitions to increase loading speed of the list

[Didayolo]

Following #1173. The competitions list page is slow to load:

https://www.codabench.org/competitions/public/?page=1

This is because the logos of the competitions can be big images. We load all images just to display small thumbnail (e.g. 200 x 200 pixels).

A possible solution is to save a new attribute competition.logo_small, during unpacking of bundle, with a resized version of the logo. This thumbnail logo would be used in the competition list page.

[Didayolo]

Another take: resize the image (and keep only one logo as it is already the case).
We can limit the width to 400 pixels.

[bbearce]

#1306