This solves two problems: first, the relationships are now defined in
the class they apply to, rather than in a separate section of the module,
and second, their metadata -- both creation arguments and extra info such
as `description` (or, later, possibly, info for API properties) -- is
stored.
No reason to instantiate every time as_html's called, is there?
Also, sessions use a markdown_extension attribute instead of
markdown_extension_class. The latter is only used to set the former when
the session is created (unless another markdown_extension_class is given,
of course).
Previously, every single spline-pokedex request tacked another markdown
extension onto a global list in spline, making markdown processing just
a little bit slower over time. This is terrible.
Now we do something a little less crazy and a little more global. Wait,
is that less crazy or more?
- the Session has a `pokedex_link_maker` property, whose `object_url`
method is used to make URLs in Markdown
- pokemon.names_table.name is now an ordinary Unicode column
- pokemon.name is a MarkdownString that is aware of the session and the
language the string is in
- pokemon.name_map is a dict-like association_proxy of the above
- move.effect works similarly, with transparent $effect_chance substitution
as before
