Use the new `settings.hostPageSetting()` method in `index.js`.

This means that `index.js` no longer uses `shared/settings.js`
(`jsonConfigsFrom()`) or `configFuncSettingsFrom()`. These are both
encapsulated in `hostPageSetting()`.

Previously `index.js` blindly assigned a lot of property names and
values fo the `config` object using a couple of `Object.assign()` calls:

    Object.assign(config, sharedSettings.jsonConfigsFrom(window_.document));
    Object.assign(config, configFuncSettingsFrom(window_));

These `Object.assign()`s are now gone, and instead it explicitly assigns
each property to the `config` object by name:

    var config = {
      app: settings.app,
      query: settings.query,
      ...
      openLoginForm: settings.hostPageSetting('openLoginForm'),
      openSidebar: settings.hostPageSetting('openSidebar'),
      ...
    };

(Some of these settings were already being assigned in this way, but the
ones that uses `hostPageSetting()` are new.)

This also means that `index.js` no longer uses `isBrowserExtension()`.
Previously `index.js` contained logic to return early, declining to read
various config settings from the host page using its `Object.assign()`s,
if the client was running in a browser extension. This logic is also now
encapsulated in the new `settings.hostPageSetting()` method and so is
removed from `index.js`.