forum: add forum listing routes

With this change, URLs for forum listings are now available. This
includes URLs like /news or /projects/games. Each of them show a
(currently empty) forum index.

Note that URLs that are not linked to in the forum index, namely URLs
for forums that have children (eg. /forum/news), are still accessible. We
could ban this by raising ValidationError if the forum has a non-empty
[sub_forums] attribute but displaying all subjects feels better.

URLs that point to /forum, but do not name a sub-forum and are not of
the form of a topic URL produce 404 errors.

app/routes/forum/index.py

@@ -3,6 +3,10 @@ from app.models.forum import Forum
 from app import app
 
 @app.route('/forum')
-def forum():
+def forum_index():
     main_forum = Forum.query.filter_by(parent=None).first()
     return render('/forum/index.html', main_forum=main_forum)
+
+@app.route('/forum/<forum:f>')
+def forum_page(f):
+    return render('/forum/forum.html', f=f)

app/templates/forum/forum.html

@@ -0,0 +1,11 @@
+{% extends "base/base.html" %}
+
+{% block title %}
+<a href='/forum'>Forum de Planète Casio</a> » <h1>{{ f.name }}</h1>
+{% endblock %}
+
+{% block content %}
+<section>
+	{{ f.descr }}
+</section>
+{% endblock %}

app/utils/converters.py

@@ -1,17 +1,38 @@
 """
 utils.converter: Custom URL converters to match patterns in @app.route()
+
+The Flask documentation is elusive on this topic. To add a new converter,
+proceed as follows:
+
+1. Define a new converter class.
+2. Set the [regex] attribute to decide which portion of the URL will be
+   considered for conversion (apparently the default is everything until next
+   slash or end of string).
+3. Define the to_python() and to_url() methods to actually convert.
+4. Add the class to __all__ at the bottom of this file.
+5. In app/__init__.py, add a dictionary entry to [app.url_map.converters].
+
+For more information, see the Werkzeug documentation:
+  <https://werkzeug.palletsprojects.com/en/0.15.x/routing/#custom-converters>
 """
 
-from werkzeug.routing import BaseConverter
+from werkzeug.routing import BaseConverter, ValidationError
 from app.models.forum import Forum
 import re
+import sys
 
 class ForumConverter(BaseConverter):
 
+    # This regex will decide which portion of the URL is matched by the curtom
+    # converter. By default, slashes are not included, so we must add them.
+    regex = r'[a-z/]+'
+
     def to_python(self, url):
+        print(f"ForumConverter url from '{url}'", file=sys.stderr)
+        url = '/' + url
         f = Forum.query.filter_by(url=url).first()
         if f is None:
-            raise Exception(f"ForumConverter: no forum with url {value}")
+            raise ValidationError(f"ForumConverter: no forum with url {url}")
         return f
 
     def to_url(self, forum):
@@ -19,9 +40,12 @@ class ForumConverter(BaseConverter):
 
 class TopicSlugConverter(BaseConverter):
 
+    # Only catch integers followed by an optional slug string
+    regex = r'(\d+)(?:-[\w-]*)?'
+
     def to_python(self, url):
         """Convert an URL pattern to a Python object, or raise an exception."""
-        m = re.fullmatch(r'(\d+)(?:-[\w-]*)?', url)
+        m = re.fullmatch(TopicSlugConverter.regex, url)
         if m is None:
             raise Exception(f"TopicSlugConverter: conversation failed")