Move default apps to plugins.entry_points

• The default apps (entries, calculations, materials, elns) are currently defined in config.ui. They should be moved to plugins.entry_points (similar to the built-in example uploads) for now.
• Currently removing the entries app will break several pages, as they rely on the presence of that app. Instead, a default app should be included directly in the GUI code.
• The word 'api' is missing in the pluralization dictionary: should be added.