Multiple attachments feature

[teners]

Just in case, we want to be able to attach multiple files to a post.

In the current DB schema a post has just picture: ImageField field, so...

Also, we need to discuss and think through the logic of attachments itself. Say, for instance, it's possible in VK to attach a photo and a gif to a post, but Twitter doesn't support that — should be either a gif, or photos.