Reduce app store screenshot size for list view

[BernhardPosselt]

Hi, the current app store upload features a screenshot (https://github.com/strukturag/nextcloud-spreedme/blob/master/screenshots/appstore/conference.gif) which is 2.1Mb big.

It would be cool if someone could reduce the size so that the app list loads faster. The preferred way is to use the small-thumbnail attribute for the preview image, see https://github.com/nextcloud/news/blob/master/appinfo/info.xml#L25

It's fine if you want to stick with a gif, but you should set a static image on the app overview or go with APNG (see http://caniuse.com/#search=apng), the important thing is setting the small-thumbnail attribute :)