[5.2] Other JS improvements

[C-Lodder]

Summary of Changes

1. Convert site template JS to ES6
2. Remove [].slice.call() calls where possible.
3. Other minor improvements

Testing Instructions

1. Ensure the front-end MetisMenu is working properly.
2. Code review @dgrammatiko @Fedik

[HLeithner]

@dgrammatiko @Fedik can you have a look?

[HLeithner]

This pull request has been automatically rebased to 5.2-dev.

[Fedik]

I have tested this item :white_check_mark: successfully on 559efc8f34defed37c9b2bff2121aea272a250ad

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[viocassel]

I have tested this item :white_check_mark: successfully on 559efc8f34defed37c9b2bff2121aea272a250ad

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[richard67]

@C-Lodder I've allowed myself to solve the conflict in file build/media_source/system/js/inlinehelp.es6.js which was caused by your other PR #42726 having been merged up meanwhile. I hope I've done it all right.

[richard67]

RTC as the previous human tests are still valid. The commit which invalidated the test count was just a branch update with a small easy conflict which I've resolved.

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[C-Lodder]

Thanks @richard67

[pe7er]

Thanks @C-Lodder !