replace tera with askama and eliminate all unused features

Originally Askama was used as the template engine in Ayllu and then it was
replaced with Tera so that themes packages could change page layouts. This
was interesting but too complicated and so I've reverted back to using Askama.

All theme customization is handled by simple CSS files now and two example
themes: nord and catppuccin have been added. Nord is now the default theme
used by Ayllu.

Reverting back to Askama required many refactorings and all unused features
have been eliminated from the UI. The blame page has also been eliminated
since it never looked very good and I don't really have the skills to implement
a nice UI/UX for it.

The dependency on npm to download the open-props package has been eliminated
and the site uses all custom CSS. The lightningcss build.rs script has also
been eliminated and we simply concatentate a base CSS file and theme CSS file
together. A future change should try to minimize the CSS but the whole thing
is like 20kb and so I don't really care.

Stateful services, Maitred SMTP, and the Papyri container registry will
continue to be developed separately until they're completely ready to be
integrated into Ayllu.