add: thumbnail with icon for posts without media or with spoiler

[plebeius-eth]

Every post on reddit has a thumbnail, I think Seedit should use these thumbnail icons as well. They align every post content, so they are also helpful on mobile. The spoiler one uses Snoo so we shouldn't use it, I think the spoiler label is enough.

https://t.me/seeditreact/7665

[plebeius-eth]

also all thumbnails should have a width of 70px just like on old reddit

[plebeius-eth]

72d77fa

[plebeius-eth]

also added the spoiler one, but showing a simple exclamation point icon instead of snoo 851e0e8

[plebeius-eth]

7785039