Auto select frontend for links to "parent" service

Farside now supports redirecting based on a provided link to a "parent"
service, if such a parent service is supported.

For example, a link such as:

farside.link/https://www.youtube.com/watch?v=dQw4w9WgXcQ

will now redirect to any of the available YouTube related frontends.

This works by matching against a mapping of "parent" service domains
("youtube.com", "reddit.com", etc) to a list of their respective frontend
alternatives (["invidious", "piped"], ["libreddit", "teddit"], etc). A
random element is chosen from this list, and the remainder of Farside's
routing logic proceeds as if the user had chosen the service directly to
begin with.

Closes benbusby#37

Update throttle test to reflect new behavior

The "throttling" performed by Farside now redirects to the same instance
as before, rather than outright blocking the connection.

[CI] Auto update instances

Allow bypassing app router with `FARSIDE_NO_ROUTER`

Setting the aforementioned env var skips creation of the app router,
which is useful for running update.exs when the main app is already
running (otherwise there's a port conflict).