Upgrade stack from LTS 17.2 to 18.18

This required bumping nixpkgs. No code changes were required.

We now include stack in the nix environment, so that we don't have
to depend on stack being installed externally.

Author: rkrzr

client-haskell/default.nix

@@ -11,4 +11,5 @@ let
 in
   {
     icepeak-client = icepeak-client;
+    stack = pkgs.haskellPackages.stack;
   }

client-haskell/stack.yaml

@@ -1,4 +1,4 @@
-resolver: lts-17.2
+resolver: lts-18.8
 
 # Note: This section will be ignored by stack, on non-NixOS systems.
 # It can be explicitly enabled on non-NixOS systems by passing --nix.

nix/nixpkgs.nix

@@ -1,8 +1,8 @@
 let
-  rev = "bed08131cd29a85f19716d9351940bdc34834492";
+  rev = "d6a4500f88725c24b82f6f86fb3129ed0561800c";
   extractedTarball = fetchTarball {
     url = "https://github.com/NixOS/nixpkgs/archive/${rev}.tar.gz";
-    sha256 = "sha256:19gxrzk9y4g2f09x2a4g5699ccw35h5frznn9n0pbsyv45n9vxix";
+    sha256 = "sha256:1xvgir3jr0mff9zk3ca2m0mzk6blyhjwmd5flyp3jp83bphr7301";
   };
 in
   # extractedTarball will be a directory here, and 'import' will automatically append /default.nix here

server/default.nix

@@ -11,4 +11,5 @@ let
 in
   {
     icepeak-server = icepeak-server;
+    stack = pkgs.haskellPackages.stack;
   }

server/package.yaml

@@ -26,10 +26,10 @@ dependencies:
 - monad-logger                      >= 0.3.31 && < 0.4
 - mtl                               >= 2.2.2 && < 2.3
 - network                           >= 2.8.0 && < 3.2
-- optparse-applicative              >= 0.14.0 && < 0.16
+- optparse-applicative              >= 0.15.0 && < 0.17
 - prometheus-client                 >= 1.0.0 && < 1.1
 - prometheus-metrics-ghc            >= 1.0.0 && < 1.1
-- random                            >= 1.1 && < 1.2
+- random                            >= 1.1 && < 1.3
 - raven-haskell                     >= 0.1.2 && < 0.2
 - scotty                            >= 0.11.5 && < 0.13
 - securemem                         >= 0.1.10 && < 0.2