Windows - User profile directory untidy, suggested improvements.

On my Windows 7 install, I've noted the User profile directory for Keybase is rather messy.

`c:\users\USERNAME\AppData\Roaming\Keybase`
contains three sub-folders (GPUCache, Local Storage and Cache), and several files I'd expect to see such as lockfile, cookies, etc.

I also have **86 files** of varying names that are in all cases image previews, or zero-byte files that were probably trying to be image previews (they had such high hopes!). Oddly, none of these are in the Cache folder, as I'd expect. And some of these are nearly a year old (not modified since April of 2017).

Suggested improvements:

1. Image previews should be stored in a sub-folder.
2. Periodically, images not modified or accessed in over NN days should be purged.
3. Image previews that could not be successfully downloaded should not leave a zero-byte file behind.
4. Please settle on a consistant naming scheme for previews and stick with it, :)

I'm also a bit hesitant about the idea of caching images from Keybase chat in a completely unencrypted form on the local filesystem. I'd hope that they'd be encrypted or only kept in memory.

Obviously, like any digital medium, Keybase can be used for criminal endeavors. I don't know what the proper ethical stance should be on if, say, a pedophile using Keybase has images of child porn cached. Is the product complicit if the end-user is busted for posession of child porn, because he/she thought Keybase secured them? That's probably a solution for your lawyers and license statements.

I personally think it'd be best to move the image preview storage to a secure medium or soley keep it in RAM. Or hey, maybe the caching of images should be in the user's KBFS. I like that idea.

HTH. HAND!

Start by inspecting the Windows Keybase profile directory at c:\users\USERNAME\AppData\Roaming\Keybase and determine how image previews are stored. Done should include organized preview storage, cleanup of old files, removal of failed zero-byte downloads, consistent naming, and a decision about encrypted or memory-only caching.

Written by the indexing model from the issue text.