WIP: Guest Uploading

[Kwonunn]

This is the first working version of the Guest Uploads feature. Sorry it's taking so long, I'm not very good at Go yet.

Make sure to build the static js files as there's a new guest.js.

There's a new Guest Tokens tab in the admin panel to manage Guest Tokens. From here you can create a guest token and get its url. On the guest upload page you can simply upload any file right now. The upload parameters are currently completely ignored, that's going to be added soon. Uploaded files will show up with default parameters as if they had been uploaded by the admin.

Feedback is very welcome, I'm sorry I'm making such a spaghetti out of it.

[Forceu]

This looks very promising, thank you very much!! Let me know once the PR is ready for review