Create IndieAuth client library + Create Micropub editor front-end

There are certain requests that are very specific to my site and my workflow that don't make sense to general-purpose Micropub clients, or that aren't as widespread as to need support. This means that if I want to create these posts, then I need to perform a manual curl, which although possible, isn't as helpful when I'm i.e. on mobile, and also means that I need to manage which access tokens are available for staging/production.

I also want to be able to more easily test things on my staging site before it going to production, which can now be easily achieved from the same frontend.

We can start by providing these:

• publishing notes with a pre-uploaded photo
• publishing replies with a title (i.e. for use with GitHub issues)
• publishing self-owned events
• publishing h-cards for https://www.jvt.me/posts/2020/03/22/at-mention-people/

Because I don't want to have to keep re-authenticating with the editor, we can provide a JWE that is encrypted with a key owned by the editor, which encrypts the access token from the given token endpoint.

We can make use of HandlerMethodArgumentResolvers to make the EditorController's implementation much more straightforward.