Add nix flake

This will add support for nix flakes. Which really helps with my
development on nixos. This has two basic parts:

1. DEVELOPMENT

After this you can use the following commands on computer with nix and a
patched version of direnv:

direnv allow
devenv up

And it will automatically start and instance of postgresql and mosquitto
for development.

And the environment variables are already configured for teslamate.

So then can start teslamate with:

npm install --prefix ./assets && npm run deploy --prefix ./assets
iex -S mix phx.server

This part is all working as designed. And pretty simple and straight
forward.

2. NIX PACKAGE OF TESLA

This is 99% complete, but some pain points I haven't addressed yet.
Because I don't yet know of good solutions.

* node2nix really does not like having "package.json" refer to file
  installed from mix. For my packages I changed it to pull the files
  from npm. But need to keep the versions in sync with mix. I filled a
  bug report on this: svanderburg/node2nix#323

* node2nix needs to be run every time assets/assets/package-lock.json is
  updated. Yuck.

* There is a checksum in flake.nix that needs to be updated whenever
  mix.lock changes.

.envrc

@@ -0,0 +1 @@
+use flake . --impure

.gitignore

@@ -61,3 +61,5 @@ config/*.env
 
 /priv/plts/*.plt
 /priv/plts/*.plt.hash
+/.direnv
+/.devenv

assets/default.nix

@@ -0,0 +1,17 @@
+# This file has been generated by node2nix 1.11.1. Do not edit!
+
+{pkgs ? import <nixpkgs> {
+    inherit system;
+  }, system ? builtins.currentSystem, nodejs ? pkgs."nodejs_14"}:
+
+let
+  nodeEnv = import ./node-env.nix {
+    inherit (pkgs) stdenv lib python2 runCommand writeTextFile writeShellScript;
+    inherit pkgs nodejs;
+    libtool = if pkgs.stdenv.isDarwin then pkgs.darwin.cctools else null;
+  };
+in
+import ./node-packages.nix {
+  inherit (pkgs) fetchurl nix-gitignore stdenv lib fetchgit;
+  inherit nodeEnv;
+}