discourse: Fix plugin support

For plugins to work properly, their assets need to be precompiled
along with the rest of Discourse's assets. This means we need to build
new packages when the list of plugins change.

(cherry picked from commit 9af3672f4faaafba0ce0129a87fc7925c14eeb61)

nixos/modules/services/web-apps/discourse.nix

@@ -30,6 +30,9 @@ in
       package = lib.mkOption {
         type = lib.types.package;
         default = pkgs.discourse;
+        apply = p: p.override {
+          plugins = lib.unique (p.enabledPlugins ++ cfg.plugins);
+        };
         defaultText = "pkgs.discourse";
         description = ''
           The discourse package to use.
@@ -731,8 +734,6 @@ in
 
           cp -r ${cfg.package}/share/discourse/config.dist/* /run/discourse/config/
           cp -r ${cfg.package}/share/discourse/public.dist/* /run/discourse/public/
-          cp -r ${cfg.package}/share/discourse/plugins.dist/* /run/discourse/plugins/
-          ${lib.concatMapStringsSep "\n" (p: "ln -sf ${p} /run/discourse/plugins/") cfg.plugins}
           ln -sf /var/lib/discourse/uploads /run/discourse/public/uploads
           ln -sf /var/lib/discourse/backups /run/discourse/public/backups