Allow users to edit their own content - Minimal User Identity

**Research needed.** I'm not sure if this is even possible, prior to building some sort of authentication/identity system.

Goals:

* Associate some sort of ID with a specific browser (probably separate from the `username` being added in #47) which uniquely identifies a give user/browser and associates it with content they create (_how? public/private key pair? public key signing of created content? sounds complicated! also, sounds risky to store a private key in local storage!_)
* If a certain user/browser has signed certain posts, also allow this user/browser to edit the caption (see #28) and username (see #47) associated with those posts

We may also want to limit post deletion to only the user who created the content. However, this requires discussion. Since people can post anonymously, it would be easy to infiltrate a board with loads of spam. We may want to give anyone the ability to delete any content, in lieu of any centrally manageable censorship ability.