aasg-brotlify: Make file extensions configurable

Allow packages to specify the file extensions to compress, by either
extending or overriding the default list.

aasg-brotlify: Decouple from installPhase

Move the hook to a custom phase to avoid requiring `installPhase`
to be run, as I suspect that websites don't get installed much.

Deprecate packages available in Nixpkgs

A good chunk of our packages are now available in Nixpkgs (though most
were not upstreamed by me).  Stop building them and issue a warning if
they're accessed directly (i.e. not through an overlay).

ci: Don't rely on `meta.platforms` being set

This should've been part of 30ebec151835, which applies the same change
to flake.nix.

make-job-script: Extract systemd service script builder from Nixpkgs

This can be used when you want to use `preStart`, `postStart`, or
`preStop` but also need to apply one of the special executable prefixes.

aasg-brotlify: New hook to auto-compress static websites

aasgBrotlifyHook can be passed in a derivation's `nativeBuildInputs`
to compress all HTML, CSS, and JS files in the output (as well as the
corrresponding source maps) with Brotli.  It is expected to be used by
packages that are or contain websites.

Switch all systemd services from Type=simple to Type=exec

Type "exec" waits until the service process is exec'ed before
reporting success, while "simple" succeeds right after the fork
and will not detect errors such as missing binaries or invalid
user/group.

trust-dns: Add dependency on libiconv

libiconv is implicitly provided by glibc on linux-gnu, but on other
platforms it need to be added as a dependency explicitly.  (I'm fixing
a test on macOS, but this might apply to the BSDs as well.)

ci: Configure nixos-20.09 channel explicitly

Ensure that we're using the latest revision of the channel, instead of
whatever revision the OS image was built with.  I hope it's reasonable
to expect users to be up to date.

If we end up rolling this back, we also need to rollback the previous
commit, as the sourcehut image comes with Cachix 0.5.1 which doesn't
have `watch-exec`.

ci: Use `cachix watch-exec` to upload built derivations

Replace the ls-then-diff logic we borrowed from cachix-action with
cachix's new watch-exec command, simplifying the build steps and
possibly reducing the CI time (by pushing in parallel with building,
haven't checked if it does that though).

trust-dns: 0.19.5 -> 0.20.1

daemonocle: 1.0.2 -> 1.2.2

prometheus-bird-exporter: 1.2.4 -> 1.2.5

matrix-appservice-irc: 0.23.0 -> 0.25.0

guile-json: 4.4.1 -> 4.5.2

dyndnsc: 0.5.1 -> 0.6.0

json-logging: init at 1.3.0

flake: Update inputs

Replace stdenv.lib with lib

This is a local counterpart of github:NixOS/nixpkgs#108938, which does
the same for Nixpkgs.  Generated files were not altered.

flake: Update inputs

rtl8723bu: 2020-09-04 -> 2020-11-24