More reactive, tested Masthead.

[jmchilton]

... also less broken since there are some bug fixes in here.

• Eliminate another layer of Backbone.View (the one previously in layout/masthead.js) and put the Masthead component directly in the page.
• Fix onbefeoreunload handling broken in [WIP] Porting Masthead to Vue.js #9071 I think.
• Remove more underscore and jQuery usage.
• Make config handling around brand config reactive and implement a unit test.
• Eliminate hackery around loading webhooks in different ways in different context, just load them when component is created. Place them in a data entry so the underlying menu structure can change reactively and these will remain.
• Add unit test for loading masthead webhooks.
• Make actual entries in the masthead more reactive - previously it would just be generated once in masthead.js

Builds on #9811 (which converted layout/menu.js to use a POJO instead of Backbone and #9810 which contains other masthead fixes.

[dannon]

Working well for me, thanks.

[galaxybot]

This PR was merged without a 'kind/' tag, please correct.

[jmchilton]

Thanks for the quick merge of a big change @dannon, much appreciated.