Support album descriptions and layout options, and add photo
descriptions

All modules are now plugins. The config.py calls load_plugin for each
plugin it needs (some plugins may load others automatically). Also each
plugin keeps its own template folder which gets added to the template
search path, so i.e. if the photo plugin is unloaded completely, the URL
endpoints won't work either (with the old system, since the HTML
templates still existed in the default root the endpoints would still
serve pages, just without any Python logic behind them).

Site layouts that include a login form on every page can now include
their relative URI (`request.path`) so the user can be redirected back
to the same page after login.