[ui-core] adds showReactAppPage to allow UI routing to pure frontend …

[bjornalm]

…(react) pages

What changes were proposed in this pull request?

This PR modifies the frontend router to support the loading of pure frontend apps, i.e. apps not dependent on being rendered by the server's mako files.

How was this patch tested?

Use this function if you want to show a React based application page 
for a specific url without the need to use legacy mako templates.
1. Add a new appsItem in HueSidebar.vue that links to the new url
2. Import the new React component at the top of this file.
3. Add a new object to the pageMapping array below as examplified here:  

Please review Hue Contributing Guide before opening a pull request.

[github-actions]

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[JohanAhlen]

Nice one!