Add base directory support

- Base directory can be changed in config.py from the default / to, for example, /taiko-web/
  - See tools/nginx_subdir.conf for an example nginx configuration with a base directory
- Custom error pages can be used, they can be set in config.py

templates/admin_songs.html

@@ -1,14 +1,14 @@
 {% extends 'admin.html' %}
 {% block content %}
 {% if admin.user_level >= 100 %}
-<a href="/admin/songs/new" class="side-button">New song</a>
+<a href="{{config.basedir}}admin/songs/new" class="side-button">New song</a>
 {% endif %}
 <h1>Songs</h1>
 {% for message in get_flashed_messages() %}
 <div class="message">{{ message }}</div>
 {% endfor %}
 {% for song in songs %}
-    <a href="/admin/songs/{{ song.id }}" class="song-link">
+    <a href="{{config.basedir}}admin/songs/{{ song.id }}" class="song-link">
         <div class="song">
             {% if song.title_lang.en and song.title_lang.en != song.title %}
                 <p><span class="song-id">{{ song.id }}.</span>