@afeld => curious what you think - sparklines to S3 #dontmerge

I saw the conversation in https://github.com/afeld/tricle/issues/29#issuecomment-54094018 and thought I'd chime in with a quick and dirty rebase to show some code, if you're interested in this solution, I'll PR a neater rebase.

A couple months ago we tried inline sparklines in emails, but there would be so many (40-60) in a single email, that several consecutive refreshes would be needed for them all to load and be visible. Now we upload them to S3, and there are no loading failures/problems. The images are still at a public path, albeit an obfuscated one, but at least we can still delete the sparkline images at a moment's notice.

Looks like:
![sparkline sample](https://github.com/ilyakava/tricle/blob/edit-spark-time/screenshot.png)

p.s.
I struggled getting the inline attachments to work properly with mailview so I ditched it for mailcatcher.