Add command-line tutorial (PreGameController) and link it from the lobby

Adds a black-terminal tutorial page at /game/pregame/{session} that walks
players through help, pwd, cd, ls, cat, rm and sudo before the real game
starts. Links to it are shown on both the lobby (waiting for players) and
ready-up screens so players can practice, repeatedly, before starting.

templates/pregame/default.html.twig

@@ -0,0 +1,21 @@
+<!DOCTYPE html>
+<html lang="{{ app.request.locale|default(app.request.defaultLocale|default('en')) }}">
+<head>
+    <meta charset="UTF-8">
+    <meta name="viewport" content="width=device-width, initial-scale=1.0">
+    <title>Command Line Tutorial</title>
+
+    {{ encore_entry_link_tags('app') }}
+    {{ encore_entry_link_tags('pregame') }}
+</head>
+<body>
+    <div id="game-container">
+        <div id="message-container">
+            <div class="message">No tutorial is available for this game yet.</div>
+        </div>
+    </div>
+</body>
+
+{{ encore_entry_script_tags('app') }}
+{{ encore_entry_script_tags('pregame') }}
+</html>

templates/pregame/game1.html.twig

@@ -0,0 +1,33 @@
+<!DOCTYPE html>
+<html lang="{{ app.request.locale|default(app.request.defaultLocale|default('en')) }}">
+<head>
+    <meta charset="UTF-8">
+    <meta name="viewport" content="width=device-width, initial-scale=1.0">
+    <title>Command Line Tutorial</title>
+
+    {% block stylesheets %}
+        {{ encore_entry_link_tags('app') }}
+        {{ encore_entry_link_tags('pregame') }}
+    {% endblock %}
+</head>
+<body>
+    <div id="game-container">
+        <div id="pregame-config"
+             data-session-id="{{ session.id|e('html_attr') }}"
+             style="display:none">
+        </div>
+
+        <div id="message-container">
+
+        </div>
+        <div id="input">
+            <input type="text" disabled id="input-message">
+        </div>
+    </div>
+</body>
+
+{% block javascripts %}
+    {{ encore_entry_script_tags('app') }}
+    {{ encore_entry_script_tags('pregame') }}
+{% endblock %}
+</html>