Refactor modules into a plugin-based system

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).