Updates to arrow 1.2.1
Adds PluginMeta tests.
Merges branch 'feature/upgrade-to-arrow-1-2-1' into develop
Adds FrontEndManifest and tests.
Moves frontend context from plugin meta to FrontEndManifest.
Adds callable context test.
Removes Shortcode dependency from FrontEndManifest.

* Makes the code path for posts without [spoiler] shortcodes faster.
  The manifest and hence asset loader code path gets ignored.
Adds Plugin tests.
Moves front end javascript for FrontEndManifest
Merges branch 'feature/add-front-end-manifest' into develop
Adds react and dependencies
Adds react based version
Merges branch 'feature/add-react' into develop
Misc improvements
Removes dist assets
Removes ember
Adds `js:` tasks that integrate with webpack.