Merge static_world and web_server examples

The new example is called file_server and it's basically
the same as web_server was. The name is clearer than the
original, all examples being "Web servers".

The new example is also tested and the test suite has
been refactored a little.

examples/file_server/src/file_server_app.erl

@@ -0,0 +1,29 @@
+%% Feel free to use, reuse and abuse the code in this file.
+
+%% @private
+-module(file_server_app).
+-behaviour(application).
+
+%% API.
+-export([start/2]).
+-export([stop/1]).
+
+%% API.
+
+start(_Type, _Args) ->
+	Dispatch = cowboy_router:compile([
+		{'_', [
+			{"/[...]", cowboy_static, {priv_dir, file_server, "", [
+				{mimetypes, cow_mimetypes, all},
+				{dir_handler, directory_handler}
+			]}}
+		]}
+	]),
+	{ok, _} = cowboy:start_clear(http, 100, [{port, 8080}], #{
+		env => #{dispatch => Dispatch},
+		middlewares => [cowboy_router, directory_lister, cowboy_handler]
+	}),
+	file_server_sup:start_link().
+
+stop(_State) ->
+	ok.