Support for active storage preview

[tachyons]

Fixes #1

[richardvenneman]

@tachyons thanks for the PR, it is nice that the previewer can resize the image. Let me test this.

But please note this doesn't fix #1 (show image preview right after image is selected from local.)

[tachyons]

Okay, I missed that. But since preview is to be generated using backend tool, previewing right after uploading won't be easy

[richardvenneman]

Also, in the case of an image this code is not reached. I am wondering if this code can just replace the conditional?

[tachyons]

I tried it, But got following error
ActiveStorage::UnpreviewableError: ActiveStorage::UnpreviewableError

[richardvenneman]

I see. It appears that previewable? only works with PDF and video files. For images we need #variable?. Let me do some more testing tonight before I merge your PR.

[richardvenneman]

@tachyons I rewrote the attachment previewing in bbbd3e9 and incorporated your input, thanks!