Implement or Hide 'Set thumbnail'

[gubuntu]

when editing a layer or map, I expect Set Thumbnail to update the thumbnail image everywhere it's used in the site (layer, map and metadata views) but it does not appear to do anything.

It should just run the existing create thumbnail process based on the current view (layers and extent)

child of #207

[gubuntu]

@NyakudyaA please test and comment

[NyakudyaA]

This still does not work. Also, see issue #359

[lucernae]

Probably just because of CDN. I will retest this.

[gubuntu]

set thumbnail works great on layers