This commit makes a number of improvements to the DiscourseJsProcessor:
1. Remove dependence on the out-of-date Ember template compiler from the ember-rails gem; switch to modern template compiler
2. Refactor to make use of a proper module system with `define`/`require`
3. Introduce `babel-plugin-ember-template-compilation` to enable inline hbs compilation
The `mini-loader` is upgraded to support relative lookup and `require.has`, so that these new JS packages work correctly.
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