Refactor the web app bootstrapping code

- All custom JS variables are now preloaded into the `app.data` object, rather than directly on the `app` object. This means that admin settings are available in `app.data.settings` rather than `app.settings`, etc.
- Cleaner route handler generation
- Renamed ConfigureClientView to ConfigureWebApp, though the former still exists and is deprecated
- Partial fix for #881 (strips ?nojs=1 from URL if possible, so that refreshing will attempt to load JS version again)

js/admin/src/components/SettingDropdown.js

@@ -12,7 +12,7 @@ export default class SettingDropdown extends SelectDropdown {
     props.defaultLabel = 'Custom';
 
     props.children = props.options.map(({value, label}) => {
-      const active = app.settings[props.key] === value;
+      const active = app.data.settings[props.key] === value;
 
       return Button.component({
         children: label,