Indiekit as an independent Micropub client

[paulrobertlloyd]

Right now, Indiekit only allows you to authenticate with the URL setting up in the configuration file, and it uses default/configured endpoint. This prevents Indiekit servers being usable by anyone with their own separate IndieAuth and Micropub endpoints.

Ideally, there should be a (default) option to sign in using the configured publication URL. If this is selected, then configured values endpoint values are used. However, if another URL is entered, endpoint discovery should be performed to find IndieAuth and Micropub endpoints.

Given the application communicates with itself using the Micropub API (not directly with the database), so once this remaining discovery piece is added, the rest should work without much intervention 🤞

(This is also likely a pre-requisite for #308)